14 Lymphatic and Immune Systems

The lymphatic vessels begin as open-ended capillaries, which feed into larger and larger lymphatic vessels, and eventually empty into the bloodstream. Along the way, the lymph travels through the lymph nodes, which are commonly found near the groin, armpits, neck, chest, and abdomen. Humans have about 500–600 lymph nodes throughout the body (see Figure 14.1). Several organs and tissues that participate in immunity are also part of the lymphatic system.

Did you know?

Lymphatic vessels and blood vessels are similar in structure and function. Lymph is not actively pumped by the heart, but is forced through the vessels by the movements of the body muscles. (Betts, et al., 2013).

Lymphatic Capillaries

An important function of the lymphatic system is to return the fluid (lymph) to the blood. Lymph may be thought of as recycled blood plasma. Blood pressure causes leakage of fluid from the blood capillaries, resulting in the accumulation of fluid in the interstitial space. In humans, 20 liters of plasma is released into the interstitial space of the tissues each day due to capillary leakage. The blood vessels reabsorb 17 liters of this interstitial fluid, leaving three litres in the tissues for the lymphatic system to transport back to the circulation. If the lymphatic system is damaged in some way, such as by being blocked by cancer cells or destroyed by injury, interstitial fluid accumulates in the tissue spaces, causing a condition called lymphedema.

Lymphatic capillaries, also called the terminal lymphatics, are vessels where interstitial fluid enters the lymphatic system to become lymph. Located in almost every tissue in the body, these vessels are interlaced among the arterioles and venules of the circulatory system in the soft connective tissues of the body. See Figure 14.2. Exceptions are the central nervous system, bone marrow, bones, teeth, and the cornea of the eye, which do not contain lymph vessels.

Larger Lymphatic Vessels, Trunks, and Ducts

The lymphatic capillaries empty into larger lymphatic vessels, which are similar to veins in terms of their three-tunic structure and the presence of valves. These one-way valves are located fairly close to one another, and each one causes a bulge in the lymphatic vessel, giving the vessels a beaded appearance (see Figure 14.2).

In general, superficial lymphatics, follow the same routes as veins, whereas deep lymphatic vessels of the viscera generally follow the paths of arteries. The superficial and deep lymphatics eventually merge to form larger lymphatic structures known as lymphatic trunks. On the right side of the body, the right sides of the head, thorax, and right upper limb trunks drain lymph fluid into the right subclavian vein via the right lymphatic duct (see Figure 14.3). On the left side of the body, the trunks from the remaining portions of the body drain into the larger thoracic duct, which drains into the left subclavian vein. The thoracic duct itself begins just beneath the diaphragm in the cisterna chyli.

Primary Lymphoid Organs

The primary lymphoid organs are the bone marrow and thymus gland. The lymphoid organs are where lymphocytes mature, proliferate, and are selected, which enables them to attack pathogens without harming the cells of the body.

• Bone Marrow
  • Recall that all blood cells, including lymphocytes, are formed in the red bone marrow.  The B cell undergoes nearly all of its development in the red bone marrow, whereas the immature T cell, called a thymocyte, leaves the bone marrow and matures largely in the thymus gland.

Lymphocytes develop and mature in the primary lymphoid organs, but they mount immune responses from the secondary lymphoid organs, which include the lymph nodes, spleen, and lymphoid nodules. A naïve lymphocyte is one that has left the primary organ, where it learned to function immunologically, and entered a secondary lymphoid organ where it waits to encounter an antigen against which it will mount a response (see Figure 14.5).

Lymph Nodes

Lymph nodes function to remove debris and pathogens from the lymph, and are thus sometimes referred to as the “filters of the lymph” (see Figure 14.6). Any bacteria that infect the interstitial fluid are taken up by the lymphatic capillaries and transported to a regional lymph node. Dendritic cells and macrophages within this organ internalize and kill many of the pathogens that pass through, thereby removing them from the body. The lymph node is also the site of adaptive immune responses mediated by T cells, B cells, and accessory cells of the adaptive immune system.

Did You Know?

You can live without your spleen. Do you remember the term for “surgical removal of the spleen”?

Spleen

The spleen is a vascular organ that is somewhat fragile due to the absence of a capsule. It is about 12 cm long and is attached to the lateral border of the stomach. The spleen is sometimes called the “filter of the blood” because of its extensive vascularization and the presence of macrophages and dendritic cells that remove microbes and other materials from the blood, including dying red blood cells. The spleen also functions as the location of immune responses to blood-borne pathogens.

Lymphoid Nodules

The other lymphoid tissues, the lymphoid nodules, consist of a dense cluster of lymphocytes without a surrounding fibrous capsule. These nodules are located in the respiratory and digestive tracts, areas routinely exposed to environmental pathogens.

Tonsils are lymphoid nodules located along the inner surface of the pharynx and are important in developing immunity to oral pathogens (see Figure 14.8). The tonsil located at the back of the throat, the pharyngeal tonsil, is sometimes referred to as the adenoid when swollen. Such swelling is an indication of an active immune response to infection. Tonsils have deep grooves called crypts, which accumulate all sorts of materials taken into the body through eating and breathing and actually “encourage” pathogens to penetrate deep into the tonsillar tissues where they are eliminated. A major function of tonsils is to help children’s bodies recognize, destroy, and develop immunity to common environmental pathogens so that they will be protected in their later lives. Tonsils are often removed in children who have recurring throat infections since swollen palatine tonsils can interfere with breathing and/or swallowing.

Concept Check

Tonsils are named after their locations.

• Look at the figure above and determine which anatomical structure is closely associated with each set of tonsils and was therefore used to name the tonsils, for example, the lingual tonsils are named after the tongue (lingula).
• Can you tell which structures were used to name the palatine tonsils and the pharyngeal tonsils?

Bronchus-associated lymphoid tissue (BALT) consists of lymphoid follicular structures with an overlying epithelial layer found along the bifurcations of the bronchi, and between bronchi and arteries. These tissues, in addition to the tonsils, are effective against inhaled pathogens.

Mucosa-associated lymphoid tissue (MALT) consists of an aggregate of lymphoid follicles directly associated with mucous membrane. MALT makes up dome-shaped structures found underlying the mucosa of the gastrointestinal tract, breast tissue, lungs, and eyes. Peyer’s patches, a type of MALT in the small intestine, are especially important for immune responses against ingested substances (see Figure `14.9). Peyer’s patches contain specialized cells that sample material from the intestinal lumen and transport it to nearby follicles so that adaptive immune responses to potential pathogens can be mounted.

Figure 14.10 Cooperation between Innate and Adaptive Immune Responses. The innate immune system enhances adaptive immune responses so they can be more effective. From Betts, et al., 2013. Licensed under CC BY 4.0.

Phase 1: Barrier Defenses

Barrier defenses are part of the body’s most basic innate defense mechanisms. They are not a response to infections, but rather are continuously working to protect against pathogens by preventing them from entering the body, destroying them after they enter, or flushing them out before they can establish themselves.

Barrier defenses examples:

• Skin:
  • Keratinized cells of the surface are too dry for bacteria to grow are continuously sloughed off, along with pathogens that are on their surfaces.
• Skin (sweat glands, sebaceous glands):
  • Lower pH than pathogens prefer, may contain substances that are toxic to pathogens, washing action.
• Oral Cavity (salivary glands):
  • Lysozyme is an enxyme that destroys bacteria.
• Stomach:
  • LowpH which is fatal to many pathogens.
• Mucosal:
  • Traps both microbes and debris, and facilitates their removal.
• Normal flora (nonpathogenic bacteria):
  • Prevents pathogens from growing on mucosal surfaces.

Phase 3: Adaptive Immune Response

Watch this video:

Media 14.3 Immune System, Part 2: Crash Course A&P #46 [Online video]. Copyright 2015 by CrashCourse.

Benefits of the Adaptive Immune Response

• Specificity
  • The ability to specifically recognize and mount a response against almost any pathogen.
  • Antigens, are recognized by receptors on the surface of B and T lymphocytes.

• Immunological Memory
  • The first exposure to a pathogen is called a primary adaptive response.
  • Symptoms of a first infection, called primary disease, are always relatively severe because it takes time for an initial adaptive immune response to a pathogen to become effective.
  • Upon re-exposure to the same pathogen, a secondary adaptive immune response is generated, which is stronger and faster that the primary response, often eliminating the pathogen before it can cause damage or even symptoms.
  • This secondary response is the basis of immunological memory, which gives us immunity.

• Self Recognition
  • The ability to distinguish between self-antigens, those that are normally present in the body, and foreign antigens, those that might be on a potential pathogen.
  • As T and B cells mature, there are mechanisms in place that prevent them from recognizing self-antigen, preventing a damaging immune response against the body. When these mechanisms fail, their breakdown leads to autoimmune diseases.

Lymphocytes: B Cells, T Cells, Plasma Cells

As stated above, lymphocytes are the primary cells of adaptive immune responses. These cells were introduced in the previous chapter and are summarized in the following table:

Table 14.1 Cells of the Adaptive Immune Response. From Betts, et al., 2013. Licensed under CC BY 4.0.

CELL TYPE	DESCRIPTION AND DETAILS
Plasma Cell	B cell (lymphocyte) that has been activated through exposure to an antigen and produces antibodies against that antigen (see the figure below) There are 5 classes of antibodies (IgM, IgG, IgE, IgA, IgD), each functioning in different ways: IgM promotes chemotaxis, opsonization, and cell lysis, making it a very effective antibody against bacteria at early stages of a primary antibody response IgG is the one that crosses the placenta to protect the developing fetus from disease and exits the blood to the interstitial fluid to fight extracellular pathogens IgA is the only antibody to leave the interior of the body to protect body surfaces. IgA is also of importance to newborns, because this antibody is present in mother’s breast milk (colostrum), which serves to protect the infant IgE is associated with allergies and anaphylaxis
T Cell	Different T cell types have the ability to either secrete soluble factors that communicate with other cells of the adaptive immune response or destroy cells infected with intracellular pathogen Cytotoxic T Cell (Tc) kill target cells by inducing apoptosis using the same mechanism as NK cells: killing a virally infected cell before the virus can complete its replication cycle results in the production of no infectious particles Helper T Cell (Th) release cytokines, which help to develop and regulate other immune system cells Suppressor T Cell (also called regulatory T cell) control T Cell response, in order to prevent too many T cells from being formed during an immune repsonse
Memory Cell	B cells and T cells formed during primary exposure to a pathogen (see the figure below) Remain in the body for a long time after an infection and are able to mount a fast and effective immune response to a pathogen if it is encountered a second time, preventing the pathogen from causing disease

It is clear that with some cancers, like Kaposi’s sarcoma (see Figure 14.14), for example, that a healthy immune system does a good job at controlling them. This disease, which is caused by the human herpes virus, is almost never observed in individuals with strong immune systems. Other examples of cancers caused by viruses include liver cancer caused by the hepatitis B virus and cervical cancer caused by the human papilloma virus. As these last two viruses have vaccines available for them, getting vaccinated can help prevent these two types of cancer by stimulating the immune response.

On the other hand, as cancer cells are often able to divide and mutate rapidly, they may escape the immune response, just as certain pathogens such as HIV do.

There are three stages in the immune response to many cancers:

1. 1. Elimination occurs when the immune response first develops toward tumor-specific antigens specific to the cancer and actively kills most cancer cells.
   2. Equilibrium is the period that follows, during which the remaining cancer cells are held in check.
   3. Escape of the immune response, and resulting disease, occurs because many cancers mutate and no longer express any specific antigens for the immune system to respond to.

This fact has led to extensive research in trying to develop ways to enhance the early immune response to completely eliminate the early cancer and thus prevent a later escape. One method that has shown some success is the use of cancer vaccines. These differ from other vaccines in that they are directed against the cells of one’s own body. Treated cancer cells are injected into cancer patients to enhance their anti-cancer immune response and thereby prolong survival. The immune system has the capability to detect these cancer cells and proliferate faster than the cancer cells do, thus overwhelming the cancer in a similar way as they do for viruses. Cancer vaccines are being developed for malignant melanoma and renal (kidney) cell carcinoma.

Immune Responses and Stress

In order to protect the entire body from infection, the immune system is required to interact with other organ systems, sometimes in complex ways. For example, hormones such as cortisol (naturally produced by the adrenal cortex) and prednisone (synthetic) are well known for their abilities to suppress T cell immune mechanisms, hence, their prominent use in medicine as long-term, anti-inflammatory drugs.

One well-established interaction of the immune, nervous, and endocrine systems is the effect of stress on immune health. In the human vertebrate evolutionary past, stress was associated with the fight-or-flight response, largely mediated by the central nervous system and the adrenal medulla. This stress was necessary for survival since fighting or fleeing usually resolved the problem in one way or another. It has been found that short-term stress diverts the body’s resources towards enhancing innate immune responses. This has the ability to act fast and would seem to help the body prepare better for possible infections associated with the trauma that may result from a fight-or-flight exchange.

On the other hand, there are no physical actions to resolve most modern day stresses, including short-term stressors like taking examinations and long-term stressors such as being unemployed or losing a spouse. The effect of stress can be felt by nearly every organ system, and the immune system is no exception (see Table 14.3). Chronic stress, unlike short-term stress, may inhibit immune responses even in otherwise healthy adults. The suppression of both innate and adaptive immune responses is clearly associated with increases in some diseases.

Table 14.3 Effects of Stress on Body Systems. From Betts, et al., 2013. Licensed under CC BY 4.0.

SYSTEM	STRESS-RELATED ILLNESS
Integumentary system	Acne, skin rashes, irritation
Nervous system	Headaches, depression, anxiety, irritability, loss of appetite, lack of motivation, reduced mental performance
Muscular and skeletal systems	Muscle and joint pain, neck and shoulder pain
Circulatory system	Increased heart rate, hypertension, increased probability of heart attacks
Digestive system	Indigestion, heartburn, stomach pain, nausea, diarrhea, constipation, weight gain or loss
Immune system	Depressed ability to fight infections
Male reproductive system	Lowered sperm production, impotence, reduced sexual desire
Female reproductive system	Irregular menstrual cycle, reduced sexual desire

Anatomy Labeling Activity

Medical Terms not Easily Broken into Word Parts

Lymphatic and Immune System Abbreviations

Diseases and Disorders of the Lymphatic and Immune Systems

The immune response can be under-reactive or over-reactive, leading to a state of disease. The factors that maintain immunological homeostasis are complex and incompletely understood.

The worst cases of the immune system over-reacting are autoimmune diseases in which the immune systems begin to attack cells of the patient’s own body, causing chronic inflammation and significant damage. The trigger for these diseases is often unknown, although environmental and genetic factors are likely involved. Treatments are usually based on resolving the symptoms using immunosuppressive and anti-inflammatory drugs. Figure 14.15 below provides two examples of autoimmune diseases: rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) (Betts, et al., 2013).

Overall, there are more than 80 different autoimmune diseases, which are a significant health problem in the elderly. Table 14.5 below lists several of the most common autoimmune diseases, the antigens that are targeted (autoantigen or “self” antigen), and the resulting tissue damage (Betts, et al., 2013).

Table 14.5 Autoimmune Diseases. From Betts, et al., 2013. Licensed under CC BY 4.0.

DISEASE	AUTOANTIGEN	SYMPTOMS
Celiac disease	Tissue transglutaminase	Damage to small intestine
Diabetes mellitus type I	Beta cells of pancreas	Low insulin production; inability to regulate serum glucose
Graves’ disease	Thyroid-stimulating hormone receptor (antibody blocks receptor)	Hyperthyroidism
Hashimoto’s thyroiditis	Thyroid-stimulating hormone receptor (antibody mimics hormone and stimulates receptor)	Hypothyroidism
Lupus erythematosus	Nuclear DNA and proteins	Damage of many body systems
Myasthenia gravis	Acetylcholine receptor in neuromuscular junctions	Debilitating muscle weakness
Rheumatoid arthritis	Joint capsule antigens	Chronic inflammation of joints

Lymphoma

Lymphoma was briefly discussed in the previous chapter.

Medical Terms in Context

Medical Specialties and Procedures Related to the Lymphatic and Immune Systems

Clinical Immunology/Allergy is a medical specialty that diagnoses and treats diseases of the immune system (Canadian Medical Association, 2018). For more information, please visit the Canadian Medical Association Specialty Profiles Clinical Immunology page (PDF File).

Skin testing (for allergies) is done by a clinical immunologist/allergist to identify allergens in Type I hypersensitivity. In skin testing, allergen extracts are injected into the epidermis, and a positive result of the wheal and flare response usually occurs within 30 minutes. The soft center is due to fluid leaking from the blood vessels and the redness is caused by the increased blood flow to the area that results from the dilation of local blood vessels at the site (Betts, et al., 2013).

Lymphatic System and Immune System Vocabulary

Active immunity

Immunity developed from an individual’s own immune system.

Acute inflammation

Inflammation occurring for a limited time period; rapidly developing.

Adaptive immune response

Relatively slow but very specific and effective immune response controlled by lymphocytes.

Afferent lymphatic vessels

Lead into a lymph node.

Allergens

Antigens that evoke type 1 hypersensitivity (allergy) responses.

Anaphylactic Shock

Also called anaphylaxis. An inhaled, ingested or injected (bee sting) allergen causes a significant drop in blood pressure along with contractions of smooth muscles of the airways.

Antibody

Antigen-specific protein secreted by plasma cells, immunoglobulin.

Antigen

Molecule recognized by the receptors of b and t lymphocytes.

Apoptosis

Programmed cell death.

B cells

Lymphocytes that act by differentiating into an antibody-secreting plasma cell.

Barrier defenses

Antipathogen defenses deriving from a barrier that physically prevents pathogens from entering the body to establish an infection.

Bone marrow

Tissue found inside bones, the site of all blood cell differentiation and maturation of b lymphocytes.

Bronchus-associated lymphoid tissue (balt)

Lymphoid nodule associated with the respiratory tract.

CD4 T Cells

CD4 is the receptor that HIV uses to get inside T cells and reproduce. CD4+ helper T cells play an important role in T cell immune responses and antibody responses.

Chemokine

Soluble, long-range, cell-to-cell communication molecule.

Chemotaxis

Movement in response to chemicals; a phenomenon in which injured or infected cells and nearby leukocytes emit the equivalent of a chemical “911” call, attracting more leukocytes to the site.

Chronic inflammation

Inflammation occurring for long periods of time.

Chyle

Lipid-rich lymph inside the lymphatic capillaries of the small intestine.

Cisterna chyli

Bag-like vessel that forms the beginning of the thoracic duct.

Complement

Enzymatic cascade of constitutive blood proteins that have antipathogen effects, including the direct killing of bacteria.

Crypts

Histologically, tonsils do not contain a complete capsule, and the epithelial layer invaginates deeply into the interior of the tonsil to form tonsillar crypts.

Cytokine

Soluble, short-range, cell-to-cell communication molecule.

Deep Lymphatic Vessels

Lymphatic vessels of the organs.

Efferent lymphatic vessels

Lead out of a lymph node.

Erythroblastosis fetalis

Disease of rh factor-positive newborns in rh-negative mothers with multiple rh-positive children; resulting from the action of maternal antibodies against fetal blood.

Genetic mutation that affects both t cell and b cell arms of the immune response.

Genetic Recombination

The combining of gene segments from two different pathogens.

Graft-versus-host disease

In bone marrow transplants, occurs when the transplanted cells mount an immune response against the recipient.

Histamine

Vasoactive mediator in granules of mast cells and is the primary cause of allergies and anaphylactic shock.

HIV

Human Immunodeficiency Virus. An infectious disease usually transmitted via blood or sexual fluids. It attacks the immune system and can lead to AIDS.

Hypersensitivities

Reacting to something that would not normally evoke a reaction.

Immune system

Series of barriers, cells, and soluble mediators that combine to response to infections of the body with pathogenic organisms.

Immunity

After an infection, memory cells remain in the body for a long time and can very quickly mount an immune response against the same pathogen if it tries to re-infect. This protects us from getting diseases from the same pathogen over again.

Immunological memory

Ability of the adaptive immune response to mount a stronger and faster immune response upon re-exposure to a pathogen.

Induration

A firm, raised reddened patch of skin.

Inflammation

Basic innate immune response characterized by heat, redness, pain, and swelling.

Innate immune response

Rapid but relatively nonspecific immune response.

Intercellular

Between cells.

Interferons

Early induced proteins made in virally infected cells that cause nearby cells to make antiviral proteins.

Interstitial Fluid

Fluid that has leaked out of blood capillaries into the tissue spaces.

Interstitial

Between cells of the tissues, often used interchangeably with ‘intercellular’.

Interstitial Space

Spaces between individual cells in the tissues.

Intracellular

Inside the cell membrane or within the cell.

Leukemia

A cancer involving an abundance of leukocytes. It may involve only one specific type of leukocyte from either the myeloid line (myelocytic leukemia) or the lymphoid line (lymphocytic leukemia). In chronic leukemia, mature leukocytes accumulate and fail to die. In acute leukemia, there is an overproduction of young, immature leukocytes. In both conditions the cells do not function properly.

Lymph

Fluid contained within the lymphatic system.

Lymph node

One of the bean-shaped organs found associated with the lymphatic vessels.

Lymphatic capillaries

Smallest of the lymphatic vessels and the origin of lymph flow.

Lymphatic system

Network of lymphatic vessels, lymph nodes, and ducts that carries lymph from the tissues and back to the bloodstream.

Lymphatic trunks

Large lymphatics that collect lymph from smaller lymphatic vessels and empties into the blood via lymphatic ducts.

Lymphocytes

White blood cells characterized by a large nucleus and small rim of cytoplasm.

Lymphoid nodules

Unencapsulated patches of lymphoid tissue found throughout the body.

Lymphoma

A form of cancer in which masses of malignant T and/or B lymphocytes collect in lymph nodes, the spleen, the liver, and other tissues. These leukocytes do not function properly, and the patient is vulnerable to infection.

Macrophage

Ameboid phagocyte found in several tissues throughout the body.

Mast cell

Cell found in the skin and the lining of body cells that contains cytoplasmic granules with vasoactive mediators such as histamine.

Memory t cells

Long-lived immune cell reserved for future exposure to an pathogen.

MHC

Major Histocompatibility Complex molecules, also called Human Leukocyte Antigen (HLA) are protein structures found on the outside of cells that help the immune system recognize non-self antigens.

Monocyte

Precursor to macrophages and dendritic cells seen in the blood.

Mucosa-associated lymphoid tissue (malt)

Lymphoid nodule associated with the mucosa.

Mucosal

Mucous membranes line body cavities that open to the outside world, including the respiratory tract, gastrointestinal tract, urinary tract and reproductive tracts.

Naïve lymphocyte

Mature b or t cell that has not yet encountered antigen for the first time.

Natural killer cell (nk)

Cytotoxic lymphocyte of innate immune response.

Neutrophil

Phagocytic white blood cell recruited from the bloodstream to the site of infection via the bloodstream.

Opsonization

An antibody or an antimicrobial protein binds to a pathogen, thereby marking it as a target for phagocytes.

Passive immunity

Transfer of immunity to a pathogen to an individual that lacks immunity to this pathogen usually by the injection of antibodies.

Pathogens

Disease causing agents.

Phagocytosis

Movement of material from the outside to the inside of the cells via vesicles made from invaginations of the plasma membrane.

Plasma cell

Differentiated b cell that is actively secreting antibody.

Primary adaptive response

Immune system’s response to the first exposure to a pathogen.

Primary lymphoid organ

Site where lymphocytes mature and proliferate, red bone marrow and thymus gland.

Right lymphatic duct

Drains lymph fluid from the upper right side of body into the right subclavian vein.

S. aureus

Staphylococcus aureus is a bacterium that is commonly found in minor skin infections, as well as in the nose of some healthy people.

Secondary adaptive response

Immune response observed upon re-exposure to a pathogen, which is stronger and faster than a primary response.

Secondary lymphoid organs

Sites where lymphocytes mount adaptive immune responses, examples include lymph nodes and spleen.

Seroconversion

The reciprocal relationship between virus levels in the blood and antibody levels. As the antibody levels rise, the virus levels decline, and this is a sign that the immune response is being at least partially effective (partially, because in many diseases, seroconversion does not necessarily mean a patient is getting well).

Severe combined immunodeficiency disease (scid)

Genetic mutation that affects both t cell and b cell arms of the immune response.

Spleen

Secondary lymphoid organ that filters pathogens from the blood (white pulp) and removes degenerating or damaged blood cells (red pulp).

Superficial Lymphatics

Lymphatic vessels of the subcutaneous tissues of the skin.

Systemic Lupus Erythematosus

SLE is an autoimmune disease in which the immune system recognizes its own cell antigens as being “non-self” and mounts an immune response against them. As a result, many body tissues and vital organs become chronically inflamed and damaged.

T cell

Lymphocyte that acts by secreting molecules that regulate the immune system or by causing the destruction of foreign cells, viruses, and cancer cells.

Thoracic duct

Large duct that drains lymph from the lower limbs, left thorax, left upper limb, and the left side of the head.

Thymocytes

Lymphocytes that develop into T-cells in the thymus gland.

Thymus

Primary lymphoid organ, where t lymphocytes proliferate and mature.

Tonsils

Lymphoid nodules associated with the nasopharynx.

Tissue typing

The determination of MHC molecules in the tissue to be transplanted to better match the donor to the recipient.

Vaccine

A killed or weakened pathogen or its components that, when administered to a healthy individual, leads to the development of immunological memory (a weakened primary immune response) without causing much in the way of symptoms.

Vasodilation

The smooth muscle layer in the wall of the blood vessel relaxes, allowing the vessel to widen. This decreases blood pressure in the vessel.

Wheal and flare response

A soft, pale swelling at the site surrounded by a red zone.

Test Yourself

References

Canadian Medical Association. (2018, August). Clinical immunology/allergy profile. Canadian Medical Association Specialty Profiles. https://www.cma.ca/sites/default/files/2019-01/immunology-allergy-e.pdf.

[CrashCourse]. (2015, November 30). Lymphatic system: Crash course A&P #44 [Video]. YouTube. https://youtu.be/I7orwMgTQ5I

[CrashCourse]. (2015, December 8). Immune system, part 1: Crash course A&P #45 [Video]. YouTube. https://youtu.be/GIJK3dwCWCw

[CrashCourse]. (2015, December 14). Immune system, part 2: Crash course A&P #46 [Video]. YouTube. https://youtu.be/2DFN4IBZ3rI

Image Descriptions

Figure 14.1 image description: The left panel shows a female human body, and the entire lymphatic system is shown Labels read (clockwise from top): thymus, lymph nodes, thymus, spleen, lymph vessel, bone marrow, right lymphatic duct, entering vein, tonsil, adenoid. The right panel shows magnified images of the thymus and the lymph node. Labels read (clockwise from top): tissue cell, interstitial fluid, lymphatic capillary, blood capillary, lymphatic vessel. Label of lymph node reads masses of lymphocytes and macrophages. [Return to Figure 14.1].

Figure 14.2 image description: This image shows the lymph capillaries in the tissue spaces. Labels read (clockwise, from top): lymph capillary, tissue cells, venule, lymphatic vessel, tissue fluid, arteriole). It also shows a magnified image shows the interstitial fluid and the lymph vessels. Labels read (clockwise, from top): collagen fiber, interstitial fluid, lymph, lymph vessel endothelial cells, backflow prevention valve, endothelial flaps. [Return to Figure 14.2].

Figure 14.3 image description: This figure shows the lymphatic trunks and the duct system in the human body. Labels read (clockwise from top) thoracic duct, cisterna chyli of thoracic duct, drained by thoracic duct, drained by right lymphatic duct. Callouts to the left and right show the magnified views of the left and right jugular vein respectively. Labels read (right lymphatic duct): right internal jugular vein, right subclavian vein, right lymphatic duct; (left jugular vein): left internal jugular vein, thoracic duct drains into subclavian vein, left subclavian vein.[Return to Figure 14.3].

Figure 14.4 image description: The left panel of this figure shows the head and chest of a woman and the location of the thymus is marked. Labels read (clockwise, from top) lymph nodes, spleen, heart, thymus, right lymphatic duct entering vein, tonsil, adenoid. The top right panel shows a micrograph of the thymus. Labels read (from left to right): medulla, cortex, trabeculae, fibrous capsule. The bottom right panel shows a magnified view of the structure of the thymus. Labels read (clockwise, from top): thymocytes, trabecula, fibrous capsule, cortex, medulla (layers), medullary epithelial cell, blood vessel, macrophage, dendritic cell, cortical epithelial cell.[Return to Figure 14.4].

Figure 14.5 image description:This flowchart shows the process in which a naïve T cell become activated T cells in the left part of the pathway and memory cells in the right part of the pathway. A naive T cell becomes an activated T cell when an antigen-presenting cell is introduced. The antigen is extracted from a pathogen and then either activated T cells are cloned and destroy the infected cells in the body, and/or memory T cells are produced and are activated if this antigen is encountered again. [Return to Figure 14.5].

Figure 14.6 image description: The left panel of this figure shows a micrograph of the cross section of a lymph node. Labels indicate the connective tissue capsule, cortex, and subcapsular sinus. The right panel shows the structure of a lymph node. Labels indicate (from top, clockwise) the efferent lymphatic vessels, connective tissue capsule, subcapsular sinus, cortex, afferent lymphatic vessels, trabecula, germinal centers. [Return to Figure 14.6].

Figure 14.7 image description: The top left panel shows the location of the spleen in the human body. The top center panel shows a close up view of the location of the spleen. Labels read (clockwise, from top): hilum, spleen, diaphragm, splenic vein, splenic artery. The top right panel shows the blood vessels and spleen tissue. Labels read (from left to right, top then bottom) red pulp, trabecula (bottom) white pulp, arteriole, venule. The bottom panel shows a histological micrograph Labels read (clockwise, from top): trabecula, marginal zone, central artery or arteriole, germinal center, venuous sinus, red pulp, arterial capillaries. [Return to Figure 14.7].

Figure 14.8 image description: The top panel of this image shows the locations of the tonsils. Labels read (clockwise from top):palatine tonsil, palatine bone, tongue, mandible, hyoid, trachea, esophagus. Callout shows the location of the pharyngeal tonsil. Labels read (from top): brain, sphenoidal sinus, sphenoid bone, pharyngeal tonsil, nasopharynx. Another callout details the location of the palatine tonsil. Labels read (from top): palatine tonsil, lingual tonsil, epiglottis. Another callout shows a photograph of the back of the throat where the tonsils are located. Labels read (from top) hard palate, soft palate, uvula, palatine tonsils (swollen due to infection) and tongue. The bottom panel shows the histological micrograph of the tonsils. Labels read (from top): crpyt, stratified squamous epithelium, germinal centers. [Return to Figure 14.8].

Figure 14.9 image description: This figure shows a micrograph of a mucosa associated lymphoid tissue (MAST) nodule. Labels indicate the mucosa and Peyer’s patches (which appear to be dark purple). [Return to Figure 14.9].

Figure 14.12 image description: This graph shows the antibody concentration as a function of time in primary and secondary response. Initial exposure indicates a low concentration of antibody, which then elevates over time during the primary immune response. It decreases a little during secondary exposure, but then spikes during the secondary immune response. [Return to Figure 14.12].

Figure 14.13 image description: This flow chart shows how the clonal selection of B cells takes place. The left panel shows the primary response and the right panel shows the secondary response. During a primary B cell immune response, both antibody-secreting plasma cells and memory B cells are produced. These memory cells lead to the differentiation of more plasma cells and memory B cells during secondary responses. [Return to Figure 14.13].

Figure 14.15 image description: The left panel of this figure shows an x-ray image of a person’s hand with rheumatoid arthritis, and the right panel of this figure shows a woman’s body with labels showing the different responses in the body when the patient suffers from lupus. Labels (from top, clockwise) read: psychological: fatigue, loss of appetite, face butterfly rash, pleura inflammation, pericardium inflammation, fingers and toes poor circulation, joints arthritis, muscles aches, mouth and nose ulcers, systemic: low-grade fever photosensitivity.[Return to Figure 14.51].

Unless otherwise indicated, this chapter contains material adapted from Anatomy and Physiology (on OpenStax), by Betts, et al. and is used under a a CC BY 4.0 international license. Download and access this book for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction.