STUDENT LEARNING OBJECTIVES

At the completion of this chapter, you should be able to do the following:

1.Briefly outline the components of the lymphatic and immune systems.

2.Contrast the composition of lymph with that of interstitial fluid.

3.Outline the general circulation of lymph through lymphatic vessels and nodes.

4.List several major groups of lymph nodes and their locations.

5.List the lymphatic functions of the following: tonsils, thymus, spleen.

6.Outline an overview of innate immunity.

7.List and briefly discuss the three lines of immune defense.

8.Discuss the significance of fever and inflammation.

9.Outline the roles of the following: macrophages, diapedesis, NK cells, interferon.

10.Give an overview of adaptive immunity.

11.Discuss the major types of immune system molecules and indicate how antibodies and complement proteins function.

12.Discuss the diversity of antibodies and their functions.

13.Discuss and contrast the development and functions of B and T cells.

14.Compare and contrast antibody-mediated and cell-mediated immunity.

LANGUAGE OF SCIENCE AND MEDICINE

Before reading the chapter, say each of these terms out loud. This will help you avoid stumbling over them as you read.

acquired immunity

(ah-KWYERD ih-MYOO-nih-tee) [immun- free, -ity state]

active immunity

(AK-tiv ih-MYOO-nih-tee) [actus- moving, immun- free, -ity state]

adaptive immunity

(ah-DAP-tiv ih-MYOO-nih-tee) [adapt- adjust, -ive relating to, immun- free, -ity state]

agglutination

(ah-gloo-tin-AY-shun) [agglutin- glue, -ation process]

aggregated lymphoid nodule

(ag-rah-GAYT-ed LIM-foyd NOD-yool) [a(d)- to, -grega- collect, lymph- water, -oid like, nod- knot, -ule small]

anastomosis

(ah-nas-toh-MOH-sis) [ana- anew, -stomo- mouth, -osis conditions of] pl., anastomoses (ah-nas-toh-MOH-seez)

antibody

(AN-tih-bod-ee) [anti- against]

antibody-mediated immunity

(AN-tih-bod-ee–MEE-dee-ayt-ed ih-MYOO-nih-tee) [anti- against, medi- middle, -ate process, immun- free, -ity state]

antibody titer

(AN-tih-bod-ee TYE-ter) [anti- against, titer proportion (in a solution)]

antigen

(AN-tih-jen) [anti- against, -gen produce]

antigen-antibody complex

(AN-tih-jen-AN-tih-bod-ee KOM-pleks) [anti- against, -gen produce, anti- against, complex embrace]

antigen-presenting cell (APC)

(AN-tih-jen sell) [anti- against, -gen produce, cell storeroom]

autoimmunity

(aw-toh-ih-MYOO-nih-tee) [auto- self, -immun- free, -ity state]

axillary lymph node

(AK-sih-lair-ee limf) [axilla- wing, -ary relating to, lymph water, nod- knot]

B cell

[B bursa-equivalent tissue, cell storeroom]

booster shot

CD system

(C-D SIS-tem) [C cluster, D differentiation, system organized whole]

cell-mediated immunity

(sell-MEE-dee-ayt-ed ih-MYOO-nih-tee) [cell storeroom, medi- middle, -ate process, immun- free, -ity state]

cellular immunity

(SELL-yoo-lar ih-MYOO-nih-tee) [cell- storeroom, -ular relating to, immun- free, -ity state]

chemotaxis

(kee-moh-TAK-sis) [chemo- chemical, -taxis movement]

chyle

(kile) [chyl- juice]

clone

(klohn) [clon a plant cutting]

combining site

complement

[comple- complete, -ment result of action]

constant region

(KONS-tent REE-jun)

cytokine

(SYE-toh-kyne) [cyto- cell, -kine movement]

diapedesis

(dye-ah-peh-DEE-sis) [dia- through, -pedesis an oozing]

edema

(eh-DEE-mah) [edema swelling]

effector cell

(ah-FEK-tor sell) [effect- accomplish, -or agent, cell storeroom]

effector T cell

[effect- accomplish, -or agent, T thymus gland, cell storeroom]

epitope

(EP-ih-tope) [epi- on or upon, -tope place]

fever

(FEE-ver)

germinal center

(JER-mih-nal SEN-ter) [germ- sprout, -al relating to]

hematopoietic stem cell

(hee-mah-toh-poy-ET-ik) [hema- blood, -poie- make, -ic relating to, cell storeroom]

immune system

(ih-MYOON SIS-tem) [immun- free, system organized whole]

immunization

(ih-myoo-nih-ZAY-shun) [immun- free (immunity), -tion process of]

immunoglobulin (Ig)

(ih-myoo-noh-GLOB-yoo-lin) [immuno- free (immunity), -glob- ball, -ul- small, -in substance]

inflammatory response

(in-FLAM-ah-toh-ree) [inflamm- set afire, -ory relating to]

innate immunity

(IN-ayt ih-MYOO-nih-tee) [innat- inborn, immun- free, -ity state]

interferon (IFN)

(in-ter-FEER-on) [inter- between, -fer- strike, -on substance]

interstitial fluid (IF)

(in-ter-STISH-al FLOO-id) [inter- between, -stit- stand, -al relating to]

lacteal

(LAK-tee-al) [lact- milk, -al relating to]

lingual tonsil

(LING-gwal TAHN-sil) [lingua- tongue, -al relating to]

lymph

(limf) [lymph water]

lymphatic capillary

(lim-FAT-ik KAP-ih-lair-ee) [lymph- water, -atic relating to, capill- hair, -ary relating to]

lymphatic system

(lim-FAT-ik SIS-tem) [lymph- water, -atic relating to, system organized whole]

lymphatic vessel

(lim-FAT-ik) [lymph- water, -atic relating to]

lymph node

(limf) [lymph water, nod- knot]

lymphoid tissue

(LIM-foyd TISH-yoo) [lymph- water, -oid like, tissu fabric]

macrophage

(MAK-roh-fayj) [macro- large, -phage eat]

major histocompatibility complex (MHC)

(his-toh-kom-pat-ih-BIL-ih-tee KOM-pleks) [histo- tissue, -compatibil- agreeable, -ity state, complex embrace]

mastitis

(mass-TYE-tis) [mast- breast, -itis inflammation]

membrane attack complex (MAC)

(MEM-brayne KOM-pleks) [membran- thin skin, complex embrace]

memory B cell

[B bursa-equivalent tissue, cell storeroom]

memory cell

memory T cell

[T thymus gland, cell storeroom]

naïve

(nye-EVE) [naïve natural]

naïve B cell

(nye-EVE B sell) [naïve natural, B bursa-equivalent tissue, cell storeroom]

natural killer (NK) cell

neutrophil

(NOO-troh-fil) [neuter- neither, -phil love]

nonspecific immunity

(non-speh-SIF-ik ih-MYOO-nih-tee) [non- not, -specif- form or kind, -ic relating to, immun- free, -ity state]

opsonization

(op-son-ih-ZAY-shun) [opson- condiment, -ization process]

palatine tonsil

(PAL-ah-tyne TAHN-sil) [palat- palate, -ine relating to]

passive immunity

(PAS-iv ih-MYOO-nih-tee) [immun- free, -ity state]

phagocytosis

(fag-oh-sye-TOH-sis) [phago- eating, -cyt- cell, -osis condition]

phagosome

(FAG-oh-sohm) [phago- eat, -some body]

pharyngeal tonsil

(fah-RIN-jee-al TAHN-sil) [pharyng- throat, -al relating to]

plasma cell

(PLAZ-mah sell) [plasma something molded (blood plasma), cell storeroom]

pus

radical mastectomy

(RAD-ih-kal mas-TEKtoh-mee) [radic- root, -al relating to, mast- breast, -ec- out, -tom- cut, -y action]

right lymphatic duct

(lim-FAT-ik) [lymph- water, -atic relating to]

self-tolerance

species resistance

(SPEE-sheez ree-ZIS-tens) [species form or kind]

specific immunity

(speh-SIF-ik ih-MYOO-nih-tee) [specif- form or kind, -ic relating to, immun- free, -ity state]

spleen

T cell

[T thymus gland, cell storeroom]

thoracic duct

(thoh-RAS-ik) [thorac- chest (thorax), -ic relating to]

thymocyte

(THY-moh-syte) [thymo- thyme flower (thymus gland), -cyte cell]

thymosin

(THY-moh-sin) [thymos- thyme flower (thymus gland), -in substance]

thymus

(THY-mus) [thymus thyme flower] pl., thymuses

tonsil

(TAHN-sil)

tonsillectomy

(tahn-sih-LEK-toh-mee) [tonsil- tonsil, -ec- out, -tom- cut, -y action]

tonsillitis

(tahn-sih-LYE-tis) [tonsil- tonsil, -itis inflammation]

toxoid

(TOK-soyd) [tox- poison, -oid like]

vaccination

(vak-sih-NAY-shun) [vaccin- cow (cowpox), -ation process]

variable region

(VAIR-ee-ah-bel REE-jun)

KOSTAS remembered getting the flu (influenza) last winter: coughing, fever, achiness all over his body, watery eyes, and fatigue. He felt awful! But he argued, “What's the point of a flu shot, when all it does is give you the flu?” Kostas did not understand that the injected flu vaccine is a combination of span inactivated (killed) viruses injected into muscles in your body (usually in your arm). No active viruses are injected. So, as for the injected form of the vaccine “causing” the flu—people who claim that could already have been exposed to a flu virus before the vaccination or could have been exposed to one of the strains not included in that year's vaccine. Some people produce a mild immune reaction that can be mistaken for the flu. In the end, Kostas relented and got his flu shot!

We're sure you'll enjoy reading about your own immune system, and how your body is programmed to protect you from disease. At the end of this chapter, you should be able to answer some questions about Kostas and his flu shot.

Now that you have read this chapter, see if you can answer these questions about the flu shot Kostas received in the Introductory Story.

1. Which of Kostas' cells will respond to the flu antigens introduced by the vaccine?

a. Erythrocytes

b. Thrombocytes

c. Lymphocytes

d. Platelets

2. Which specific cell types will begin producing antibodies to the antigens?

a. Z cells

b. T cells

c. A cells

d. B cells

3. Which antibody is primarily involved in this response to vaccine?

a. IgM

b. IgE

c. IgG

d. IgD

4. Kostas' fever during the previous winter's flu was caused by the release of ___, molecules that help increase his body's “set point” to higher than normal.

a. Vulcanogens

b. Histamine

c. Pyrogens

d. Nanobodies

5. What would you call the specific type of immunity Kostas developed as a result of the vaccination?

a. Natural active immunity

b. Artificial active immunity

c. Natural passive immunity

d. Artificial passive immunity

To solve a case study, you may have to refer to the glossary or index, other chapters in this textbook, A&P Connect, Mechanisms of Disease, and other resources.

COMPONENTS OF THE LYMPHATIC AND IMMUNE SYSTEMS

We have combined two related systems in this chapter: the lymphatic system and the immune system.

The lymphatic system has at least three different functions. First, it serves to maintain the fluid balance of our internal body environment. Second, the lymphatic system serves to house the immune defenses of our body. Third, the lymphatic system also helps regulate the absorption of lipids from digested food in the small intestines and provides for their transport to the large systemic veins. As you will see, the vessels of the lymphatic system roughly parallel the vessels of the cardiovascular system.

The immune system serves to repel and destroy the hordes of microorganisms that threaten our lives every day. In addition, our immune system must defend us from our own abnormal cells that can cause cancerous tumors. Such tumors may damage surrounding tissues and spread cancer throughout the body. Without an internal “security force” to deal with such abnormal cells when they first appear, we would have very short lives indeed!

LYMPHATIC SYSTEM

Overview of the Lymphatic System

Figure 19-1 gives you an excellent start to your understanding of the lymphatic system.

As you can see, plasma filters into interstitial spaces from blood flowing through the capillaries. Most of this interstitial fluid is absorbed by tissue cells or is reabsorbed by the blood before it flows out of the tissue. However, a small amount of the interstitial fluid remains behind. It seems insignificant, but if even small amounts of extra fluid continued to accumulate in the tissues over time, the result would be tremendous edema (swelling). This would be followed by tissue destruction. The lymphatic system solves the problem of fluid retention in our tissues. In fact, the entire system acts as a drainage system: It continuously collects excess tissue fluid and returns it to the venous blood just before it reaches the heart.

The lymphatic system consists of a moving fluid (lymph) derived from the blood and tissue fluid as well as a group of vessels (lymphatics) that return the lymph to the blood. In addition to lymph and lymphatic vessels, the system includes various structures that contain lymphoid tissue. This tissue, as we will see, contains lymphocytes and other defensive cells of the immune system. For example, lymph nodes are located along the paths of the collecting lymphatic vessels. Additional lymphoid tissue is found in the intestinal wall, appendix, tonsils, thymus, spleen, and bone marrow (Figure 19-2, A).

The lymphatic system provides a unique transport function. It returns tissue fluid, proteins, fats, and other substances to the general circulatory system. However, unlike the circulatory system, the lymphatic vessels do not form a closed

FIGURE 19-1 Role of the lymphatic system in fluid balance. Fluid from plasma flowing through the capillaries moves into interstitial spaces. Although most of this interstitial fluid is either absorbed by tissue cells or resorbed by capillaries, some of the fluid tends to accumulate in the interstitial spaces. As this fluid builds up, it drains into lymphatic vessels that eventually return the fluid to the venous blood.

system of vessels. Instead they begin blindly in the intercellular spaces of the soft tissue of the body (see Figure 19-1).

Lymph and Interstitial Fluid

Lymph is a clear fluid found in the lymphatic vessels, whereas interstitial fluid (IF) is the complex fluid that fills the spaces between the cells. Both lymph and interstitial fluid closely resemble blood plasma in composition. However, lymph cannot clot like blood. If the main lymphatic trunks in the thorax (see Figure 19-2) are damaged, the flow of lymph must be stopped surgically or death ensues.

Lymphatic Vessels

Distribution of Lymphatic Vessels

Lymphatic vessels begin as microscopic blind-ended lymphatic capillaries. If the lymphatic vessels originate in the villi of the small intestine, they are called lacteals (see Chapter 21). The wall of each lymphatic capillary consists of a single layer of flattened endothelial cells. We have extensive networks of lymphatic capillaries that branch and then rejoin repeatedly to form an elaborate network throughout the interstitial spaces of our bodies.

The lymphatic capillaries merge to form larger and larger vessels until main lymphatic trunks are formed. These include the right lymphatic duct and the thoracic duct seen in Figure 19-2. Lymph from the entire body, except from the upper right quadrant, eventually drains into the thoracic duct. This in turn drains into the left subclavian vein at the point where it joins the left internal jugular vein (see Figure 19-2, B). Lymph from the upper right quadrant empties into the right lymphatic duct and then into the right subclavian vein.

Note that the thoracic duct is considerably larger than the right lymphatic duct. This is because most of the body's lymph returns to the bloodstream via the thoracic duct.

Structure of Lymphatic Vessels

The walls of lymphatic capillaries have numerous openings or clefts between the cells. This makes them much more porous or permeable than blood capillaries. As lymph flows from the thin-walled lymphatic capillaries into vessels with larger diameters, the walls become thicker. Eventually these larger vessels have the three layers typical of arteries and veins.

One-way valves are abundant in lymphatic vessels of all sizes. These valves give the vessels a somewhat beaded appearance. Valves are present every few millimeters in large lymphatics and are even more numerous in the smaller vessels (Figure 19-3).

Function of Lymphatic Vessels

The lymphatics play a vital role in numerous homeostatic mechanisms. The great permeability of the lymphatic capillary wall permits very large molecules and even small particles to

FIGURE 19-2 Lymphatic system. A, Principal organs of the lymphatic system. B, The right lymphatic duct drains lymph from the upper right quadrant (dark blue) of the body into the right subclavian vein. The thoracic duct drains lymph from the rest of the body (yellow) into the left subclavian vein. The lymphatic fluid is thus returned to the systemic blood just before entering the heart.

be removed from the interstitial spaces. In fact, proteins that accumulate in the tissue spaces can return to blood only by the lymphatic system. This fact has great clinical importance. For example, if anything blocks the return of lymph for an extended period of time, blood protein concentration and blood osmotic pressure soon fall below normal. The result is fluid imbalance and death.

Lacteals from the villi of the small intestine are important in the absorption of fats and other nutrients. Chyle—the milky lymph found in lacteals after digestion—contains 1% to 2% fat.

Circulation of Lymph

Water and solutes continually filter out of capillary blood into the interstitial fluid (refer again to Figure 19-1). To balance this outflow from the system, fluid continually re-enters blood from the interstitial fluid. We now know that about 50% of the total blood protein leaks out of the capillaries into

FIGURE 19-3 Structure of a typical lymphatic capillary. Notice that interstitial fluid enters through clefts between overlapping endothelial cells that form the wall of the vessel. Valves ensure one-way flow of lymph out of the tissue.

the interstitial fluid and ultimately returns to the blood by way of the lymphatic vessels (Figure 19-4).

The Lymphatic Pump

Even without a pump like the heart, lymph moves slowly and steadily along in its lymphatic vessels into the general circulation at about 3 L/day. This occurs despite the fact that most of the flow is against gravity! It does so because of the large number of valves that permit fluid flow only in the general direction toward the heart.

Breathing movements and skeletal muscle contraction aid in this return movement of lymph to the circulatory system, just as they assist venous blood return, as we have seen (Chapter 10). During strenuous exercise, lymph flow may increase 10 to 15 times over normal because of skeletal muscle contraction. In this way, the continuous flow of lymph serves as an important homeostatic mechanism that maintains the constancy of our body fluids.

1. Where can you find lymphoid tissue?

2. Compare the composition of lymph and interstitial fluid.

3. Briefly describe the structure and function of lymphatic capillaries.

4. Briefly describe the factors that aid the movement of lymphatic fluid.

Lymph Nodes

Structure of Lymph Nodes

Lymph nodes (lymph glands) are oval-shaped or bean-shaped structures (Figure 19-5) that are distributed widely throughout the body. Some are as small as a pinhead; others are as large as a lima bean. The vast system of lymph nodes is linked together by the lymphatic vessels. Note in Figure 19-5, C, that lymph moves into a node by way of several afferent lymphatic vessels and emerges by one or two efferent vessels, creating an effective biological filter. One-way valves keep lymph flowing only in one direction.

Fibrous partitions or trabeculae extend from the covering capsule toward the center of a lymph node, creating compartments called cortical nodules. Each cortical nodule within the lymph node is composed of packed lymphocytes that surround a less dense area, the germinal center (see Figure 19-5, C). When an infection is present, germinal centers enlarge and the node begins to release lymphocytes. Special leukocytes called B lymphocytes (B cells) begin their final stages of maturation within the germinal center of the nodule. They are then pushed into the denser outer layers to mature before becoming antibody-producing plasma cells.

The center, or medulla, of a lymph node is composed of sinuses that separate medullary cords composed of plasma

FIGURE 19-4 Circulation plan of lymphatic fluid. This diagram outlines the general scheme for lymphatic circulation. Fluids from the systemic and pulmonary capillaries leave the bloodstream and enter interstitial spaces, thus becoming part of the interstitial fluid (IF). The IF also exchanges materials with the surrounding tissues. Often, because less fluid is returned to the blood capillary than had left it, IF pressure increases—causing IF to flow into the lymphatic capillary. The fluid is then called lymph (lymphatic fluid) and is carried through one or more lymph nodes and finally to large lymphatic ducts. The lymph enters a subclavian vein, where it is returned to the systemic blood plasma. Thus fluid circulates through blood vessels, tissues, and lymphatic vessels in a sort of “open circulation.”

cells and B cells. Both the cortical and medullary sinuses are lined with macrophages ready for phagocytosis.

Locations of Lymph Nodes

Most lymph nodes occur in groups, or clusters (see Figure 19-2), in certain areas, especially the head and neck. A total of approximately 500 to 600 lymph nodes are located throughout our bodies. Before you continue, take a moment to review the locations of the major lymph nodes in Figure 19-2.

Function of Lymph Nodes

Our lymph nodes defend our bodies from invading pathogens and also provide sites for the maturation of some types of lymphocytes.

Lymph flow slows as it passes through the sinus channels of the lymph nodes. This gives the special cells that line the channels time to remove microorganisms and other injurious particles. Here, the offending material is engulfed in the process of phagocytosis and destroyed. Thus, lymph nodes are the sites of both biological and mechanical filtration.

Sometimes, however, the lymph nodes are overwhelmed by massive numbers of infectious microorganisms. The nodes themselves then become sites of infection. Most people have experienced the pain of swollen lymph nodes. In addition, cancer cells breaking away from a malignant tumor may also enter the lymphatic system. They travel to the lymph nodes and may create cancerous growths that block the flow of lymph. This leaves too few channels for lymph to return to the blood and swelling results. For example, if tumors block axillary lymph node channels (located under our arms), fluid accumulates in the interstitial spaces of the arm, causing swelling and pain from the edema. Even viruses such as human immunodeficiency virus (HIV) and other types of pathogens can infect or infest lymph nodes.

Lymphoid tissues of lymph nodes also serve as the site for the final stages of maturation of some types of lymphocytes and monocytes.

Lymphatic Drainage of the Breast

Distribution of Lymphatics in the Breast

The mammary glands and surrounding tissues of the breast are drained by two sets of lymphatic vessels. There are lymphatics that originate in and drain the surface area and skin over the breast (excluding the areola and nipple areas). There are also lymphatics that originate in and drain the underlying tissue of the breast itself (including the skin of the areola and nipple).

FIGURE 19-5 Structure of a lymph node. A, A lymph node is typically a small structure into which afferent lymphatic ducts empty their lymph. Efferent lymphatic ducts drain the lymph from the node. An outer fibrous capsule maintains the structural integrity of the node. B, Photograph of a dissected cadaver shows a lymph node and its associated lymphatic vessels, along with nearby muscles, nerves, and blood vessels. C, Internal structure of a lymph node. Several afferent valved lymphatics bring lymph to the node. In this example, a single efferent lymphatic leaves the node at a concave area called the hilum. Note that the artery and vein also enter and leave at the hilum. Arrows show direction of lymph movement.

FIGURE 19-6 Lymphatic drainage of the breast. Note the extensive network of lymphatic vessels and nodes that receive lymph from the breast. Surgical procedures called mastectomies, in which some or all of the breast tissues are removed, are sometimes performed to treat breast cancer. Because cancer cells can spread so easily through the extensive network of lymphatic vessels associated with the breast, the lymphatic vessels and their nodes are sometimes also removed. Occasionally, such procedures cause swelling, or lymphedema.

More than 85% of the lymph from the breast enters the lymph nodes of the axillary region (Figure 19-6). Most of the remainder enters lymph nodes along the lateral edges of the sternum. Several very large nodes in the axillary region physically contact extensions of breast tissue.

Lymph Nodes Associated with the Breast

There are many anastomoses (connections) between the superficial lymphatics from both breasts. These anastomoses can allow cancerous cells from one breast to invade normal tissue from the other breast. Removal of a wide area of deep fascia is therefore required in surgical treatment of advanced or diffuse breast malignancy. Such a surgical procedure is called a radical mastectomy. Cancer of the breast is one of the most common forms of malignancy in women. However, it can also be found (although rarely) in men.

Breast infections are also a serious health concern, especially among women who nurse their infants. Mastitis, for example, is an inflammation of the mammary gland, usually caused by infectious agents. Breast infections, like cancer, can also spread easily through lymphatic pathways associated with the breast.

5. Describe the overall structure of a typical lymph node.

6. How are lymph nodes generally distributed in your body?

7. What vital functions are performed by the lymph nodes?

8. How does the distribution of lymphatics in breast tissue and adjacent tissues relate to breast cancer and its spread?

Structure and Function of the Tonsils

Masses of lymphoid tissue, called tonsils, form a protective ring under the mucous membranes in the mouth and back of the throat (Figure 19-7). This ring of tonsils protects us against bacteria that may invade tissue in the area around the openings between the nasal and oral cavities. The palatine tonsils are located on each side of the throat. The pharyngeal tonsils (called adenoids when they become swollen) are near the posterior opening of the nasal cavity. A third type of tonsil, the lingual tonsils, lie near the base of the tongue. Other smaller tonsils are located near the opening of the auditory (eustachian) tube. Each tonsil has deep recesses that trap bacteria and expose them to the immune system.

FIGURE 19-7 Location of the tonsils. Small segments of the roof and floor of the mouth have been removed to show the protective ring of tonsils (pharyngeal lymphoid ring) around the internal openings of the nose and throat.

The tonsils are part of our first line of defense from the external environment. As such, they are subject to chronic infection, or tonsillitis. In these cases, tonsils may be surgically removed (tonsillectomy), if non-surgical treatments prove ineffective. However, because of the critical immunological role played by the lymphatic tissue, the number of tonsillectomies performed annually continues to decrease.

Structure and Function of Aggregated Lymphoid Nodules

Aggregated lymphoid nodules, also called Peyer patches, are groups of small oval patches or groups of lymph nodes that form a single protective layer in the mucous membrane of the small intestine, especially the ileum. Because the entire gastrointestinal tract is potentially open to the external environment via the mouth, these patches are in a great location to provide immune surveillance in an area where massive numbers of potentially pathogenic bacteria can be found. The macrophages and other cells of the immune system prevent most of these bacteria from penetrating the gut wall. Aggregated lymphoid nodules and other lymphoid tissues are sometimes called mucosa-associated lymphoid tissue (MALT).

Structure and Function of the Thymus

The thymus is a primary organ of the lymphatic system. It consists of two pyramid-shaped lobes. The thymus is located in the mediastinum, extending up into the neck, close to the thyroid gland (Figure 19-8). It is largest (relative to body size) in a child about 2 years old. After puberty, however, the thymus gradually atrophies. In advanced old age, it may be largely replaced by fat, becoming yellow in color.

FIGURE 19-8 Thymus. Location of a child's thymus within the mediastinum.

The thymus plays a critical part in the body's defenses against infection. Before birth, the thymus serves as the final site of lymphocyte development. Many lymphocytes leave the thymus and circulate to the spleen, lymph nodes, and other lymphoid tissue.

Soon after birth, the thymus assumes another function. It begins secreting a group of peptide hormones (collectively called thymosin) and other regulators that enable lymphocytes to develop into mature T cells. Only T lymphocytes that pass “immunological testing” are released into the bloodstream.

Structure and Function of the Spleen

The spleen is located below the diaphragm, just above most of the left kidney and behind the fundus of the stomach (see Figure 19-2). Roughly oval in shape (Figure 19-9), the spleen varies somewhat in size from individual to individual. For example, it enlarges (hypertrophies) during infectious disease and shrinks (atrophies) in old age.

FIGURE 19-9 Structure of the spleen. Medial aspect of the spleen. Notice the concave surface that fits against the stomach within the abdominopelvic cavity.

The spleen has a variety of functions, including defense, hematopoiesis, and erythrocyte and platelet destruction. It also serves as a reservoir for blood.

Defense is accomplished as blood passes through highly permeable, enlarged blood vessels, called sinusoids. Macrophages lining these venous spaces remove microorganisms from the blood and destroy them by phagocytosis. Hematopoiesis takes place when monocytes and lymphocytes complete their development and become “activated” in the spleen.

Before birth, red blood cells are also formed in the spleen. However, after birth, the spleen forms red blood cells only in cases of severe anemia. Macrophages lining the spleen's sinusoids remove worn-out red blood cells and imperfectly formed platelets. Macrophages also break apart the hemoglobin molecules from the destroyed red blood cells. They salvage iron and globin content from destroyed erythrocytes and return these by-products of destruction to the bloodstream. From here, they are sent to storage in the bone marrow and liver.

Finally, the spleen and its venous sinuses hold a considerable amount of blood. This blood reservoir can rapidly be added back into the circulatory system if it is needed. However, if the spleen is ruptured, as when the ribs are broken and pushed into the spleen, significant internal bleeding can occur, followed by death. Surgical rep