Ch. 20: The Lymphatic System

Overview of the Lymphatic System

The lymphatic system is a network of tissues and organs that help rid the body of toxins, waste, and other unwanted materials. Its primary function is to transport lymph, a fluid containing infection-fighting white blood cells, throughout the body.

• Lymphatic Circulation: Lymph is collected from tissues and returned to the bloodstream, maintaining fluid balance and contributing to immune defense.

• Functions: Includes fluid recovery, immunity, and lipid absorption.

• Pathway of Lymph: Lymph flows from lymphatic capillaries to lymphatic vessels, through lymph nodes, and eventually drains into the cardiovascular system via the thoracic duct or right lymphatic duct.

Lymphatic Tissues and Organs

• Peyer's Patches: Aggregated lymphoid nodules found in the small intestine, important for monitoring intestinal bacteria and preventing the growth of pathogenic bacteria.

• Lymph Nodes: Small, bean-shaped structures that filter lymph and house lymphocytes. Anatomy includes cortex (with follicles) and medulla.

• MALT (Mucosa-Associated Lymphoid Tissue): Lymphatic tissues found in mucosal linings (e.g., gut, respiratory tract) that protect against pathogens.

• Spleen: Largest lymphatic organ; filters blood, removes old red blood cells, and mounts immune responses.

• Thymus: Site of T cell maturation; most active during childhood and shrinks with age.

• Tonsils: Lymphatic tissue located in the pharynx; types include palatine, pharyngeal (adenoids), and lingual tonsils.

Functions of the Lymphatic System

• Immune Defense: Houses and transports immune cells, filters pathogens from lymph.

• Fluid Balance: Returns excess interstitial fluid to the bloodstream.

• Lipid Absorption: Absorbs dietary lipids from the small intestine via lacteals.

Example: Pathway of Lymph

• Lymphatic capillaries → lymphatic vessels → lymph nodes → lymphatic trunks → lymphatic ducts → subclavian veins

Ch. 21: The Immune System

Innate and Adaptive Immunity

The immune system protects the body from infectious agents and other foreign substances. It is divided into innate (nonspecific) and adaptive (specific) immunity.

• Innate Immunity: First line of defense; includes physical barriers (skin, mucous membranes), phagocytic cells, and inflammation.

• Adaptive Immunity: Specific response involving lymphocytes (B and T cells) and the production of antibodies.

• Natural Killer (NK) Cells: Lymphocytes that destroy virus-infected and tumor cells without prior sensitization.

• Opsonization: Process by which pathogens are marked for ingestion and destruction by phagocytes.

Cells and Molecules of the Immune System

• B Cells: Respond to antigens by differentiating into plasma cells that secrete antibodies.

• Plasma Cells: Effector B cells that produce antibodies.

• Antibodies: Immunoglobulins (Ig) that bind specific antigens; five types include IgG, IgM, IgA, IgE, and IgD.

• T Cells: Include helper T cells (CD4+), cytotoxic T cells (CD8+), and regulatory T cells; involved in cell-mediated immunity.

• Regulatory T Cells: Suppress immune responses to prevent autoimmunity.

Immune Responses and Pathways

• Primary vs. Secondary Immune Response: Primary response occurs upon first exposure to antigen; secondary response is faster and stronger due to memory cells.

• Active vs. Passive Immunity: Active immunity results from direct exposure to antigen; passive immunity is acquired through transfer of antibodies (e.g., maternal antibodies).

• Complement Activation: Classical pathway involves antibody binding to antigen, triggering complement proteins to lyse pathogens.

• Membrane Attack Complex (MAC): Structure formed by complement proteins that creates pores in pathogen membranes, leading to cell lysis.

• Phagocyte Mobilization: Movement of phagocytic cells to sites of infection during inflammation.

Major Histocompatibility Complex (MHC)

• MHC Class I: Present on all nucleated cells; present endogenous antigens to cytotoxic T cells.

• MHC Class II: Present on antigen-presenting cells; present exogenous antigens to helper T cells.

Key Terms and Definitions

• Antigen: Any substance that can induce an immune response.

• Hapten: Small molecule that can elicit an immune response only when attached to a larger carrier.

Example: Classical Pathway of Complement Activation

• Antibody binds to antigen → C1 complex binds to antibody → Activation of C2 and C4 → Formation of C3 convertase → Cleavage of C3 → Opsonization, inflammation, and cell lysis

Additional info:

• Adaptive immunity includes both humoral (antibody-mediated) and cell-mediated responses.

• Vaccines work by stimulating active immunity, leading to the formation of memory cells.