2.11: Alternate Text Descriptions

Panel (a): World Map of HIV/AIDS Prevalence

This panel displays a global map in which countries are color-coded according to the percentage of the population living with HIV/AIDS.

• Dark red indicates the highest prevalence, greater than 20%, concentrated primarily in southern Africa.
• Red and orange shades indicate prevalence levels between approximately 5% and 20%, also concentrated largely in sub-Saharan Africa.
• Yellow indicates prevalence levels between approximately 0.5% and 2%.
• Light green indicates prevalence levels between approximately 0.1% and 0.5%.
• Yellow-green indicates prevalence levels below 0.1%.
• Gray-green indicates countries categorized as AIDS-free. Examples include Australia, New Zealand, and Japan.
• Pale blue-gray indicates No data available. An example shown on the map is Greenland.

Overall, the map highlights that HIV/AIDS prevalence is substantially higher in parts of Africa than in most other regions of the world. Southern Africa contains the highest concentrations of countries with prevalence rates above 15% and 20%, while much of North America, Europe, Asia, South America, and Oceania have substantially lower prevalence rates.

Panel (b): Microscopic Image of HIV Virus and Immune Cell

This panel is a color-enhanced microscopic image showing HIV particles interacting with a human immune cell.

• A large blue structure represents a human immune cell, likely a T lymphocyte.
• Pink, irregular structures extend from the cell surface and represent portions of the cell membrane and membrane projections.
• Numerous small green spherical particles surrounding the cell represent HIV virions.

The image illustrates HIV particles attaching to and interacting with immune cells, an important step in viral infection. HIV targets immune system cells and uses them to reproduce, ultimately impairing the body's ability to fight infections. Together, the two panels emphasize both the global impact of HIV/AIDS and the cellular interactions that occur during HIV infection.

Main Body: Lymphatic System Overview

The central image shows a full-body view of the lymphatic system. A network of lymphatic vessels extends throughout the body from the head to the feet. Small lymph nodes are distributed along these vessels, particularly in the neck, armpits, chest, abdomen, and groin.

Several major lymphatic organs are identified:

• The adenoids are located at the back of the nasal cavity in the upper throat.
• The tonsils are located at the back of the mouth and throat.
• The thymus is located in the upper chest, directly above the heart and behind the sternum.
• The spleen is located in the upper left side of the abdomen beneath the rib cage.
• Bone marrow is shown within a long bone of the leg, illustrating the location where many blood and immune cells are produced.
• The right lymphatic duct is shown emptying lymph into a large vein near the base of the neck, where lymph re-enters the bloodstream.

The figure emphasizes that lymphatic vessels collect fluid from tissues throughout the body and return it to the circulatory system.

Enlarged View: Thymus

An enlarged image of the thymus is shown adjacent to the body. This view highlights the thymus as a lobed organ located in the upper chest. The thymus is a primary lymphatic organ where T lymphocytes mature before entering circulation and participating in immune responses.

Enlarged View: Formation and Movement of Lymph

A circular magnified view shows the relationship between tissue cells, blood capillaries, interstitial fluid, lymphatic capillaries, and lymphatic vessels.

Blood capillaries are located among body tissues and release fluid into the spaces surrounding tissue cells. This fluid becomes interstitial fluid, which bathes and nourishes the cells. Excess interstitial fluid enters nearby lymphatic capillaries, where it is then called lymph. The lymph is collected into larger lymphatic vessels and transported through the lymphatic system before eventually returning to the bloodstream.

This view illustrates the pathway:

Blood capillaries → interstitial fluid surrounding tissue cells → lymphatic capillaries → lymphatic vessels

Enlarged View: Lymph Node

A second enlarged view shows a cross-section of a lymph node connected to lymphatic vessels.

The lymph node contains dense collections of lymphocytes and macrophages, concentrated within the interior of the node. These immune cells monitor and filter lymph as it passes through the node, helping remove pathogens, abnormal cells, and foreign material.

This view emphasizes the immune function of lymph nodes as filtration and surveillance sites within the lymphatic system.

The figure demonstrates how the lymphatic system collects excess fluid from body tissues, transports that fluid through lymphatic vessels and lymph nodes, and ultimately returns it to the bloodstream. It also highlights several major lymphatic organs, including the adenoids, tonsils, thymus, spleen, and bone marrow, which contribute to immune system function and the production, maturation, or storage of immune cells. Unlike the cardiovascular system, the lymphatic system does not have a central pump; lymph movement depends on body movement, skeletal muscle contraction, breathing, and one-way valves that prevent backflow and help direct lymph toward the veins of the neck.

Tissue-Level View

The left portion of the figure shows tissue spaces located between body cells. An arteriole supplies blood to a network of blood capillaries within the tissue, and a venule collects blood leaving the capillary bed.

As blood passes through the capillaries, some fluid leaves the bloodstream and enters the spaces between tissue cells. This fluid is called interstitial fluid or tissue fluid. The interstitial fluid surrounds and bathes the tissue cells, providing nutrients and removing wastes.

Interspersed throughout the tissue spaces are branching lymphatic capillaries, which collect excess interstitial fluid. These capillaries merge into larger lymphatic vessels, which transport the collected fluid through the lymphatic system toward the bloodstream.

This portion of the figure demonstrates the pathway:

Blood capillaries → interstitial fluid surrounding tissue cells → lymphatic capillaries → lymphatic vessels

Enlarged View of a Lymphatic Capillary

The enlarged view shows the wall of a lymphatic capillary and the mechanism by which interstitial fluid enters the lymphatic system.

The lymphatic capillary is located among tissue cells and is surrounded by interstitial fluid. The wall of the capillary is composed of overlapping lymph vessel endothelial cells that form flexible endothelial flaps.

Collagen fibers anchor the lymphatic capillary to the surrounding tissue. These fibers help prevent collapse of the capillary and assist in opening the endothelial flaps when tissue pressure rises.

Excess interstitial fluid moves from the spaces between tissue cells through openings between the endothelial flaps and enters the lymphatic capillary. Once the fluid enters the capillary, it is called lymph.

Within the lymphatic capillary, the overlapping endothelial flaps function as one-way entry points. The structure identified as a backflow prevention valve helps prevent lymph from moving back into the surrounding tissues, ensuring that lymph continues to move in one direction through the lymphatic system.

The figure demonstrates how excess fluid that leaves blood capillaries is collected from body tissues and returned to circulation through the lymphatic system. Interstitial fluid becomes lymph after entering lymphatic capillaries through specialized endothelial openings. Collagen fibers, endothelial flaps, and one-way valves help ensure efficient fluid collection and directional movement of lymph from tissues into larger lymphatic vessels.

Main Body: Lymphatic Drainage Regions

The central figure shows a human body divided into two drainage territories.

The right lymphatic duct drains lymph from:

• The right side of the head and neck
• The right upper limb
• The right side of the thorax

The remaining regions of the body drain into the thoracic duct, including:

• The left side of the head and neck
• The left upper limb
• The left side of the thorax
• The abdomen
• The pelvis
• Both lower limbs

The thoracic duct begins in the abdomen at an enlarged sac-like structure called the cisterna chyli, which collects lymph from the lower body before the lymph travels superiorly through the thoracic duct toward the neck.

Right Lymphatic Duct

An enlarged view of the right side of the neck shows the right lymphatic duct near the junction of the right internal jugular vein and the right subclavian vein.

The right internal jugular vein descends from the head and neck, while the right subclavian vein drains blood from the right upper limb. The right lymphatic duct empties lymph into the bloodstream where these veins join, allowing lymph collected from the upper right quadrant of the body to re-enter circulation.

Thoracic Duct

A second enlarged view shows the termination of the thoracic duct on the left side of the neck.

The thoracic duct ascends through the thorax and empties lymph into the venous circulation near the junction of the left internal jugular vein and the left subclavian vein. The left internal jugular vein drains blood from the head and neck, and the left subclavian vein drains blood from the left upper limb.

Through this connection, lymph collected from most of the body is returned to the bloodstream.

The figure demonstrates that lymph collected from tissues throughout the body is ultimately returned to the venous circulation through one of two major lymphatic ducts. The right lymphatic duct drains only the upper right portion of the body, whereas the thoracic duct drains the remainder of the body. The cisterna chyli serves as a collection reservoir for lymph from the lower body before it enters the thoracic duct and is transported to the veins of the neck.

Panel A: Cross Section of the Spleen

A body reference image shows the spleen located in the upper left region of the abdomen, beneath the diaphragm and protected by the lower rib cage. The enlarged anatomical view shows the spleen adjacent to the diaphragm.

The hilum is located on the medial surface of the spleen and serves as the entry and exit point for blood vessels. The splenic artery enters the spleen through the hilum and supplies oxygenated blood, while the splenic vein exits through the hilum and carries blood away from the organ.

An internal cross-sectional view of the spleen shows two major tissue regions:

• White pulp, which consists of dense lymphatic tissue organized around small arteries and is involved in immune surveillance and immune responses.
• Red pulp, which occupies most of the spleen and is responsible for filtering blood, removing aged or damaged red blood cells, and storing blood components.

Fibrous trabeculae extend inward from the capsule of the spleen, providing structural support and carrying blood vessels through the organ.

Within the splenic tissue, small arterioles deliver blood into the spleen, while venules collect blood that has passed through the organ.

Panel B: Magnified Spleen Histology

The lower panel provides a microscopic view of splenic tissue, illustrating the detailed organization of the white pulp and red pulp.

A central artery (arteriole) runs through the white pulp region. Surrounding the central artery is a germinal center, which contains proliferating lymphocytes involved in immune responses. Encircling the germinal center is the marginal zone, a transition area where blood-borne antigens encounter immune cells.

The surrounding red pulp occupies most of the tissue and contains networks of blood-filled spaces and immune cells. Within the red pulp are venous sinuses, which collect and transport filtered blood, and arterial capillaries, which distribute blood throughout the splenic tissue.

A trabecula is visible extending through the tissue, providing structural support and serving as a pathway for blood vessels.

The figure demonstrates the relationship between the gross anatomy and microscopic organization of the spleen. The spleen receives blood through the splenic artery, filters that blood through the white pulp and red pulp, supports immune responses through lymphocyte-rich regions of the white pulp, removes aged or damaged blood cells within the red pulp, and returns filtered blood to circulation through venous pathways and the splenic vein.

Panel A: Locations of the Tonsils

The upper portion of the figure illustrates the locations of the pharyngeal, palatine, and lingual tonsils within the head and throat.

A sagittal section of the head and neck shows the palatine tonsil located on each side of the oropharynx, between the oral cavity and the throat. Nearby anatomical structures include:

• The palatine bone, which forms part of the roof of the mouth and separates the oral and nasal cavities.
• The tongue, located on the floor of the oral cavity anterior to the pharynx.
• The mandible, which forms the lower jaw and supports the lower teeth.
• The hyoid bone, located inferior to the tongue and superior to the larynx.
• The trachea, located anteriorly and carrying air toward the lungs.
• The esophagus, located posterior to the trachea and carrying food toward the stomach.

An enlarged view of the upper pharynx shows the pharyngeal tonsil located in the roof and posterior wall of the nasopharynx, behind the nasal cavity. Surrounding structures include:

• The brain, located superior to the nasopharynx.
• The sphenoidal sinus, located within the sphenoid bone above the nasopharynx.
• The sphenoid bone, located at the base of the skull.

A second enlarged view compares the positions of the palatine tonsil and lingual tonsil within the throat. The palatine tonsil is located along the lateral walls of the oropharynx, whereas the lingual tonsil is located at the base of the tongue. The epiglottis is positioned posterior to the tongue and superior to the larynx.

A clinical photograph shows the visible structures of the oral cavity. The hard palate forms the anterior roof of the mouth, while the soft palate forms the posterior roof. The uvula hangs from the posterior edge of the soft palate. The palatine tonsils, shown enlarged because of infection, are located on either side of the back of the throat. The tongue occupies the floor of the oral cavity.

Panel B: Histology of the Palatine Tonsil

The lower panel shows a microscopic view of palatine tonsil tissue.

The outer surface is lined by stratified squamous epithelium, which forms a protective covering exposed to the oral cavity.

Deep folds of the epithelial surface extend inward to form crypts. These crypts increase the surface area of the tonsil and help trap microorganisms and foreign material that enter through the mouth or throat.

Beneath the epithelium are numerous germinal centers, which are regions of densely packed lymphocytes. These germinal centers are sites of immune-cell proliferation and activation during immune responses.

The figure demonstrates the locations of the major tonsils that form part of the body's first line of immune defense against inhaled or ingested pathogens. The pharyngeal tonsil is located in the nasopharynx, the palatine tonsils are located on either side of the oropharynx, and the lingual tonsil is located at the base of the tongue. The histology of the palatine tonsil shows crypts and germinal centers that increase exposure to foreign material and support immune surveillance and immune responses.

Step 1

Untrained (−−) thymocytes from bone marrow enter the thymic cortex.

The process begins with immature thymocytes that originate in the bone marrow. These cells enter the cortex of the thymus and are identified as CD4− CD8−, meaning they express neither the CD4 receptor nor the CD8 receptor.

Step 2

Thymic epithelial cells present MHC proteins to the untrained thymocytes. Thymocytes that successfully bind with the MHC proteins continue while those that cannot undergo apoptosis. This is positive selection (selecting those that bind).

In the cortex of the thymus, thymic epithelial cells display MHC proteins. The figure shows an MHC molecule on a thymic epithelial cell interacting with an immature thymocyte receptor.

• If the immature thymocyte receptor successfully binds the MHC protein, the thymocyte survives.
• If the MHC protein does not bind the thymocyte receptor, the thymocyte undergoes apoptosis and is eliminated.

This process is called positive selection because cells capable of recognizing self-MHC molecules are selected to survive.

Step 3

Thymocytes that survive positive selection become double positive (+ +) because they receive both a CD4 receptor and a CD8 receptor.

The surviving thymocytes now express both CD4 and CD8 receptors and are identified as CD4+ CD8+. The figure shows both receptors on the cell surface as the thymocytes move from the cortex toward the medulla of the thymus.

Step 4

The surviving thymocytes move to the thymic medulla where they encounter antigen-presenting cells (APCs). These APCs present self-antigens from other body cells. Thymocytes that bind with the self-antigens undergo apoptosis, as this indicates that they will attack body cells. This is called negative selection (selection for cells that don't bind).

In the medulla of the thymus, dendritic cells functioning as antigen-presenting cells display self-antigens derived from body tissues.

The figure shows two possible outcomes:

• If neither the CD4 receptor nor the CD8 receptor binds the self-antigen, the thymocyte survives.
• If either the CD4 receptor or the CD8 receptor binds the self-antigen, the thymocyte undergoes apoptosis.

This process is known as negative selection because self-reactive cells are eliminated, promoting self-tolerance and helping prevent autoimmunity.

Step 5

Cell signals cause the self-tolerant thymocytes to express either the CD4 or the CD8 receptor, but not both. Those that express the CD4 receptor become helper T cells (TH) while those that express the CD8 receptor become cytotoxic T cells (TC).

The surviving self-tolerant thymocytes lose one of the two receptors and become single-positive cells:

• CD4+ CD8− thymocytes differentiate into mature helper T cells (TH).
• CD4− CD8+ thymocytes differentiate into mature cytotoxic T cells (TC).

After maturation, these cells leave the thymus and enter the lymphatic system, where they circulate and participate in adaptive immune responses.

The figure demonstrates how developing T cells are selected for two critical properties. Through positive selection in the thymic cortex, T cells must be able to recognize the body's own MHC molecules. Through negative selection in the medulla, T cells that react strongly to self-antigens are eliminated. The remaining self-tolerant T cells mature into either helper T cells or cytotoxic T cells and are released into the lymphatic system to participate in immune defense.

IgM (Pentamer)

• Heavy chain: μ (mu)
• Number of antigen-binding sites: 10
• Molecular weight: 900,000 daltons
• Percentage of total antibody in serum: 6%
• Crosses placenta: No
• Fixes complement: Yes
• Fc region binds to: None Shown
• Function:
  • Main antibody of the primary immune response
  • Most effective antibody for complement activation
  • Monomer form of IgM functions as a B-cell receptor

IgG (Monomer)

• Heavy chain: γ (gamma)
• Number of antigen-binding sites: 2
• Molecular weight: 150,000 daltons
• Percentage of total antibody in serum: 80%
• Crosses placenta: Yes
• Fixes complement: Yes
• Fc region binds to: Phagocytes
• Function:
  • Main antibody of secondary immune responses
  • Neutralizes toxins
  • Promotes phagocytosis through opsonization

Secretory IgA (Dimer)

• Heavy chain: α (alpha)
• Number of antigen-binding sites: 4
• Molecular weight: 385,000 daltons
• Percentage of total antibody in serum: 13%
• Crosses placenta: No
• Fixes complement: No
• Fc region binds to: None Shown
• Function:
  • Secreted into mucus, tears, saliva, and colostrum
  • Provides protection at mucosal surfaces and in body secretions

IgE (Monomer)

• Heavy chain: ε (epsilon)
• Number of antigen-binding sites: 2
• Molecular weight: 200,000 daltons
• Percentage of total antibody in serum: 0.002%
• Crosses placenta: No
• Fixes complement: No
• Fc region binds to: Mast cells and basophils
• Function:
  • Antibody involved in allergic reactions
  • Participates in defense against parasitic infections

IgD (Monomer)

• Heavy chain: δ (delta)
• Number of antigen-binding sites: 2
• Molecular weight: 180,000 daltons
• Percentage of total antibody in serum: 1%
• Crosses placenta: No
• Fixes complement: No
• Fc region binds to: None Shown
• Function:
  • Functions as a B-cell receptor

The figure demonstrates that the five immunoglobulin classes differ in structure, abundance, and function. IgG is the most abundant antibody in serum and is the only class shown crossing the placenta. IgM has the greatest number of antigen-binding sites and is the strongest activator of complement. Secretory IgA is specialized for protection of mucosal surfaces and body secretions. IgE is associated with allergic responses and antiparasitic immunity, while IgD primarily functions as a receptor on B lymphocytes.

Type I: IgE-Mediated Hypersensitivity

An allergen binds to allergen-specific IgE antibodies attached to the surface of a mast cell through Fc receptors. Crosslinking of the IgE antibodies triggers mast-cell degranulation, resulting in the release of inflammatory mediators.

The figure states: IgE is bound to mast cells via its Fc portion. When an allergen binds to these antibodies, crosslinking of IgE induces degranulation.

Examples shown include localized and systemic anaphylaxis, seasonal allergies including hay fever, food allergies such as shellfish and peanut allergies, hives, and eczema.

Type II: IgG-Mediated Cytotoxic Hypersensitivity

IgG antibodies bind to antigens located on the surface of a target cell. The figure shows two mechanisms by which the target cell can be damaged.

Antibody-dependent cellular cytotoxicity (ADCC): A cytotoxic cell bearing Fc receptors recognizes antibody-coated target cells and destroys them.

Complement activation: Binding of antibodies to surface antigens activates the complement system, resulting in cell damage.

The figure states: Cells are destroyed by bound antibody, either by activation of complement or by a cytotoxic T cell with an Fc receptor for the antibody (ADCC).

Example shown: Red blood cells destroyed by complement and antibody during a transfusion of mismatched blood type or during erythroblastosis fetalis.

Type III: Immune Complex-Mediated Hypersensitivity

Circulating antigen-antibody complexes, shown as free-floating immune complexes, become deposited in tissues. These deposits activate the complement system and attract neutrophils, resulting in inflammation and tissue injury.

The figure states: Antigen-antibody complexes are deposited in tissues, causing activation of complement, which attracts neutrophils to the site.

Examples shown include glomerulonephritis, rheumatoid arthritis, and systemic lupus erythematosus.

Type IV: Cell-Mediated Hypersensitivity

Unlike the other hypersensitivity types, Type IV reactions do not involve antibodies. A sensitized Th1 cell recognizes antigen and releases cytokines. These cytokines activate macrophages and cytotoxic T cells, leading to inflammation and tissue damage. An activated macrophage is depicted as a major effector cell in the reaction.

The figure states: Th1 cells secrete cytokines, which activate macrophages and cytotoxic T cells and can cause macrophage accumulation at the site.

Examples shown include contact dermatitis, tuberculin reaction, and autoimmune diseases such as diabetes mellitus type I, multiple sclerosis, and rheumatoid arthritis.

The figure compares four distinct mechanisms of hypersensitivity. Type I reactions involve IgE-mediated mast-cell degranulation. Type II reactions involve IgG antibodies that destroy antibody-coated cells through complement activation or ADCC. Type III reactions involve deposition of antigen-antibody complexes in tissues, causing complement activation and neutrophil recruitment. Type IV reactions are mediated by Th1 cells, macrophages, and cytotoxic T cells through cytokine signaling without antibody involvement. Together, the four pathways illustrate the major mechanisms by which immune responses can cause allergic, autoimmune, and inflammatory tissue damage.

Panel (a): Hand X-Ray Showing Rheumatoid Arthritis

The left panel is an X-ray image of both hands. The bones of the wrists, palms, and fingers are visible. The joints of multiple fingers show significant deformity, including joint narrowing, displacement, and abnormal angulation of the fingers. Several fingers appear bent away from their normal alignment, and the joint spaces are irregular. The changes are most pronounced in the small joints of the hands and fingers.

The image demonstrates the destructive effects of chronic rheumatoid arthritis on synovial joints. Persistent autoimmune inflammation damages cartilage, bone, and supporting connective tissues, leading to pain, loss of joint function, stiffness, and visible deformities of the hands.

Panel (b): Systemic Effects of Systemic Lupus Erythematosus (SLE)

The right panel shows a full-body anatomical illustration highlighting multiple organs and body systems affected by lupus. Labels identify common signs and symptoms throughout the body.

Placenta Structure

The left side of the figure shows the placenta connecting the fetal and maternal circulations.

The umbilical artery and vein carry blood between the fetus and the placenta. Within the placenta, branches of the fetal circulation are enclosed by the embryonic chorion, a membrane that normally separates fetal blood from maternal blood.

Surrounding the chorion is the maternal blood pool, which receives blood from a placental arteriole (maternal) and drains into a placental venule (maternal). Although nutrients, gases, and wastes are exchanged across the placental barrier, fetal and maternal blood are normally kept separate by the embryonic chorion.

First Exposure: Birth of First Rh⁺ Infant

The upper-right panel illustrates the first exposure of an Rh-negative mother to Rh-positive fetal blood.

The panel identifies:

• Infant blood
• Rh⁺ infant erythrocytes
• Embryonic chorion
• Rh⁻ maternal blood
• Maternal Rh antibody

Step 1: During birth, Rh⁺ fetal erythrocytes leak into maternal blood after breakdown of the embryonic chorion, which normally isolates fetal and maternal blood.

The figure shows fetal red blood cells crossing from the infant circulation into the maternal circulation.

Step 2: Maternal B cells are activated by the Rh antigen and produce large amounts of anti-Rh antibodies.

The figure shows activated maternal B cells releasing antibodies directed against the Rh antigen. Because this antibody production occurs after fetal blood enters the maternal circulation, the first Rh-positive infant is usually not harmed.

Second Exposure: Rh⁺ Fetus

The lower-right panel illustrates a later pregnancy involving another Rh-positive fetus.

The panel identifies:

• Infant blood
• Rh⁺ infant erythrocytes
• Embryonic chorion
• Rh⁻ maternal blood
• Maternal Rh antibody

Step 1: Rh antibody titer in the mother's blood is elevated after the first exposure.

The figure shows numerous anti-Rh antibodies already present within the maternal circulation as a result of immune memory from the first pregnancy.

Step 2: Rh antibodies are small enough to cross the embryonic chorion and attack the fetal erythrocytes.

Unlike fetal red blood cells, maternal antibodies can pass through the placental barrier. The figure shows anti-Rh antibodies crossing the embryonic chorion, binding to Rh-positive fetal erythrocytes, and targeting them for destruction.

The figure demonstrates the development of Rh incompatibility. During the first pregnancy, exposure to Rh-positive fetal erythrocytes sensitizes an Rh-negative mother and stimulates production of anti-Rh antibodies. During a subsequent pregnancy with an Rh-positive fetus, preexisting maternal anti-Rh antibodies cross the placenta and attack fetal red blood cells, potentially causing hemolytic disease of the newborn (erythroblastosis fetalis). The figure emphasizes the role of maternal immune memory and the ability of antibodies, but not fetal red blood cells, to cross the placental barrier.