Study Guide: The Lymphatic System and Immune System (Chapters 20 & 21)

The Lymphatic System and Lymphoid Organs and Tissues

Overview of the Lymphatic System

The lymphatic system is a network of vessels, tissues, and organs that returns leaked fluid to the blood and provides the anatomical basis for the body's defenses. It includes lymphatic vessels, lymph, and lymphoid organs such as lymph nodes, spleen, MALT, and thymus.

• Lymphatic vessels: Return fluid from tissues to the bloodstream, forming a one-way system toward the heart.

• Lymph: Fluid collected from interstitial spaces, containing proteins and other large molecules.

• Lymph nodes: Principal lymphoid organs that filter lymph and house lymphocytes.

• Spleen: Largest lymphoid organ, removes old red blood cells and pathogens from blood.

• MALT: Mucosa-associated lymphoid tissue guards entryways against pathogens.

• Thymus: Site of T lymphocyte maturation.

Lymphatic Vessels

Lymphatic vessels begin as highly permeable capillaries in loose connective tissue, allowing large molecules to enter. Lymph flows through collecting vessels, trunks, and ducts, eventually returning to the circulatory system via the thoracic or right lymphatic duct.

• Mechanisms of lymph transport: Skeletal muscle compression, breathing-induced pressure changes, and valves prevent backflow.

• Distribution: Lymphatic capillaries are widespread, except in the central nervous system, bones, and teeth.

Lymphoid Cells and Tissues

Lymphoid tissues are found in lymphoid organs and connective tissue throughout the body, providing sites for lymphocyte proliferation and immune surveillance.

• Lymphocytes: Arise in red bone marrow; mature into T cells (thymus) or B cells (bone marrow).

• Macrophages: Phagocytes that activate T lymphocytes.

• Dendritic cells: Activate T lymphocytes.

• Reticular cells: Produce stroma, supporting other cell types.

• Diffuse lymphoid tissue: Found in most organs.

• Lymphoid follicles: Aggregations such as Peyer's patches in the intestine.

Lymph Nodes

Lymph nodes filter lymph, removing microorganisms and debris before returning it to the bloodstream. They are surrounded by a fibrous capsule and contain a stroma of reticular fibers.

• Structure: Lymph enters via afferent vessels and exits via fewer efferent vessels, allowing time for immune cells to act.

• Function: Defensive filtering and immune cell activation.

Spleen

The spleen is located in the left abdominal cavity and removes old red blood cells, platelets, and foreign matter from blood. It also provides a site for lymphocyte proliferation and immune surveillance.

• Structure: Surrounded by a fibrous capsule; contains white pulp (lymphocytes) and red pulp (macrophages).

• Function: Blood filtration and immune response.

MALT (Mucosa-Associated Lymphoid Tissue)

MALT consists of lymphoid tissues in mucous membranes throughout the body, guarding entryways against pathogens.

• Tonsils: Form a ring around the pharynx, removing pathogens from food and air.

• Peyer's patches: Aggregated lymphoid nodules in the small intestine.

• Appendix: Contains lymphoid follicles off the large intestine.

Thymus

The thymus is the site of T lymphocyte maturation and secretes hormones for immunocompetence. It differs from other lymphoid organs by lacking B cells and follicles, and its stroma consists of epithelial cells.

• Structure: Thymic lobules with cortex and medulla.

• Function: T cell education and hormone secretion.

Developmental Aspects

Lymphatic vessels and nodes develop from lymph sacs budding from veins by the fifth week of embryonic development. The thymus is an endodermal derivative; other lymphoid organs derive from mesenchyme.

The Immune System: Innate and Adaptive Body Defenses

Overview of the Immune System

The immune system defends the body from disease-causing organisms and cancerous cells through innate and adaptive defenses. Innate defenses provide immediate, nonspecific protection, while adaptive defenses target specific antigens and have memory.

Innate Defenses

Surface Barriers

Skin and mucous membranes act as the first line of defense, providing physical and chemical barriers to pathogens.

• Skin: Keratinized epithelial membrane blocks entry.

• Mucous membranes: Trap pathogens.

• Protective chemicals: Acid, lysozyme, mucin, defensins, and specialized secretions inhibit or destroy microorganisms.

Innate Internal Defenses

Cells and chemicals provide the second line of defense, including phagocytes, natural killer cells, inflammation, antimicrobial proteins, and fever.

• Phagocytes: Neutrophils and macrophages engulf and digest pathogens.

• Natural killer cells: Lyse cancer and virally infected cells.

• Inflammation: Prevents spread, disposes of debris, alerts adaptive immunity, and initiates repair. Cardinal signs: redness, heat, swelling, pain.

• Antimicrobial proteins: Interferons and complement proteins attack microorganisms and hinder reproduction.

• Fever: Systemic response to infection, induced by pyrogens.

Adaptive Defenses

Adaptive immunity recognizes and destroys specific antigens, is systemic, and has memory. It consists of humoral (antibody-mediated) and cellular (cell-mediated) responses.

• Humoral immunity: Antibodies produced by B lymphocytes target extracellular antigens.

• Cellular immunity: T lymphocytes directly attack infected or abnormal cells.

Antigens

Antigens are substances that trigger adaptive defenses. Complete antigens have immunogenicity and reactivity; haptens are incomplete antigens. Antigenic determinants are specific regions that bind antibodies or lymphocytes.

• Self-antigens: Identified by MHC proteins; not antigenic to self, only to others.

B and T Lymphocytes and Antigen-Presenting Cells

B and T lymphocytes originate in bone marrow and mature in primary lymphoid organs. They develop immunocompetence and self-tolerance through selection processes. Antigen-presenting cells (APCs) activate T cells by presenting antigen fragments.

• B cells: Mature in bone marrow; produce antibodies.

• T cells: Mature in thymus; attack infected cells.

• APCs: Dendritic cells, macrophages, and B cells present antigens to T cells.

Humoral Immunity

Antibodies are produced by plasma cells derived from B lymphocytes. Clonal selection creates effector and memory cells. Immunological memory allows faster, stronger responses upon re-exposure.

• Active immunity: Body mounts response (infection or vaccination).

• Passive immunity: Preformed antibodies are given (maternal or artificial).

• Antibody structure: Four polypeptide chains; variable and constant regions.

• Antibody classes: IgM, IgG, IgA, IgD, IgE.

• Functions: Neutralization, agglutination, precipitation, complement activation.

Cellular Immunity

T lymphocytes direct adaptive immunity or attack cellular targets. CD4 cells become helper or regulatory T cells; CD8 cells become cytotoxic T cells. Activation requires antigen presentation via MHC proteins and co-stimulatory signals.

• Helper T cells: Activate B cells, T cells, and macrophages.

• Cytotoxic T cells: Directly attack infected or cancerous cells.

• Regulatory T cells: Suppress immune responses.

• Cytokines: Chemical signals that amplify immune responses.

Immune System Disorders

Immunodeficiencies, autoimmune diseases, and hypersensitivities result from insufficient or overactive immune responses.

• Immunodeficiencies: SCID, AIDS.

• Autoimmune diseases: Loss of self-tolerance; treated by immune suppression.

• Hypersensitivities: Allergies (immediate), subacute, and delayed reactions.

Developmental Aspects of the Lymphatic and Immune Systems

Stem cells originate in the liver and spleen during embryonic development, then bone marrow takes over. Lymphocytes develop immunocompetence in thymus and bone marrow, and efficiency declines with age.

Lymphoid Organ	Main Function	Cell Types
Lymph Node	Filter lymph, immune surveillance	Lymphocytes, macrophages
Spleen	Filter blood, remove old RBCs	Lymphocytes (white pulp), macrophages (red pulp)
MALT	Guard entryways	Lymphocytes, follicles
Thymus	T cell maturation	T lymphocytes, epithelial cells

Antibody Class	Main Function
IgM	First antibody produced, agglutination
IgG	Main antibody in blood, crosses placenta
IgA	Found in mucous membranes, secretions
IgD	B cell receptor
IgE	Allergic responses, parasitic infections

Additional info: Recent discoveries highlight new roles for immune cells in fat metabolism, signaling, and social behavior. Library research topics include lymphedema, autoimmune diseases, transplant surgery, and vaccine pros/cons.