Ch. 20 Immunity: Lymphatic System and Immune Responses

Ch. 20 Immunity

Immunity Overview

The immune system is a complex network of cells, tissues, and organs that defends the body against internal and external threats, such as pathogens and abnormal cells. Unlike other organ systems, it is not confined to a single organ but is distributed throughout the body, especially within the blood and lymphatic systems.

• Key Point: The immune system includes specialized cells and proteins that identify and neutralize harmful agents.

• Key Point: The lymphatic system is a major component, providing both structural and functional support to immunity.

Lymphatic System and Immunity

Structures of the Lymphatic System

The lymphatic system consists of lymphatic vessels, lymphoid tissues, and organs. It plays a crucial role in fluid balance, fat absorption, and immune defense.

• Lymphatic Vessels: Network of vessels that transport lymph (a fluid containing immune cells).

• Lymphoid Tissues and Organs: Include lymph nodes, nodules, spleen, thymus, and bone marrow.

Functions of the Lymphatic System

• Drainage: Drains interstitial fluid and leaked plasma proteins back to the blood.

• Fat Transport: Transports dietary fats from the digestive tract to the bloodstream.

• Protection: Protects the body from disease by housing and transporting immune cells.

Lymphatic Capillaries and Vessels

• Lymphatic Capillaries: Blind-ended vessels that collect excess tissue fluid.

• Lymphatic Vessels: Carry lymph toward the heart, equipped with valves to prevent backflow.

• Lymphatic Trunks: Five main trunks drain lymph into two major ducts: the right lymphatic duct and thoracic duct.

• Right Lymphatic Duct: Drains right upper body.

• Thoracic Duct: Drains all remaining regions.

• Both ducts empty into subclavian veins, returning lymph to the circulatory system.

Lymphatic Circulation

• Movement of lymph is aided by overlapping endothelial cells, valves, skeletal muscle contraction, and respiratory pump.

• Lymph flows in one direction: from tissues to the venous circulation.

Lymphatic Organs

• Primary Lymphatic Organs: Sites where lymphocytes are produced and mature.

  • Bone Marrow: Produces all blood cells, including B lymphocytes (B cells mature here).

  • Thymus: Located in the mediastinum; T lymphocytes (T cells) mature here. Most active in children, atrophies after adolescence.

• Secondary Lymphoid Organs: Sites where mature lymphocytes become activated and initiate immune responses.

  • Lymph Nodes: Filter lymph and trap pathogens.

  • Spleen: Filters blood, removes pathogens and old erythrocytes.

  • Mucosa-Associated Lymphatic Tissue (MALT): Includes tonsils and other lymphoid tissues along mucous membranes.

Spleen

• Largest mass of lymphoid tissue in the body.

• Located inferior to the stomach, in the left upper quadrant (LUQ).

• White Pulp: Contains mostly lymphocytes (T and B cells) and some macrophages.

• Red Pulp: Filled with red blood cells (RBCs), platelets, and white blood cells (WBCs).

Immunity: Innate and Adaptive

Innate Immunity (Nonspecific Defense)

Innate immunity provides the first and second lines of defense against pathogens. It is present at birth and responds rapidly to a wide range of invaders.

• First Line of Defense:

  • Physical Barriers: Skin and mucous membranes prevent entry of pathogens.

  • Chemical Barriers: Secretions (tears, saliva, sweat, gastric juices) contain antimicrobial substances.

  • Mechanical Barriers: Actions like urination, defecation, vomiting, and vaginal excretions expel microbes.

• Second Line of Defense:

  • Antimicrobial Substances: Interferons (from virus-infected cells), complement proteins (enhance immune function), and other chemicals inhibit pathogen spread.

  • Natural Killer (NK) Cells: Lymphocytes that destroy infected or abnormal cells by releasing cytotoxic molecules.

  • Phagocytosis: Neutrophils and macrophages ingest and destroy invaders in three phases: chemotaxis, adherence, and ingestion.

  • Inflammation: Localized tissue response characterized by redness, swelling, heat, and pain. Involves vasodilation, increased permeability, and migration of phagocytes.

  • Fever: Elevated body temperature inhibits pathogen growth and accelerates tissue repair.

Adaptive Immunity (Specific Defense)

Adaptive immunity is the third line of defense, providing specific responses to particular pathogens. It involves the recognition of antigens and the development of immunological memory.

• Cell-Mediated Immunity: Involves T lymphocytes (helper T cells and cytotoxic T cells) that target infected or abnormal cells.

• Antibody-Mediated (Humoral) Immunity: Involves B lymphocytes that produce antibodies to neutralize pathogens in body fluids.

• Memory: Both T and B cells develop memory cells for faster, stronger responses upon re-exposure to the same antigen.

Clonal Selection and Antigen Processing

Antigens and Antigen Receptors

• Antigen: Any substance recognized as foreign by the immune system, typically large molecules like proteins or polysaccharides.

• Antigenic Determinant (Epitope): Specific region of an antigen that is recognized by immune cells.

• Self-Antigens: Major histocompatibility complex (MHC) proteins mark cells as "self." There are two main classes:

  • Class I MHC: Present on all nucleated cells.

  • Class II MHC: Present on antigen-presenting cells (APCs).

Endogenous Antigens

• Produced within the body (e.g., viral proteins, cancer cell proteins).

• Presented on cell surfaces by class I MHC molecules.

• Signal for destruction by cytotoxic T cells.

Exogenous Antigens

• Originate outside the body (e.g., bacteria, toxins).

• Processed and presented by antigen-presenting cells (APCs) via class II MHC molecules to helper T cells.

T Cells: Helper and Cytotoxic

Helper T Cells (CD4+)

• Recognize antigens presented by APCs with class II MHC.

• Secrete cytokines to activate other immune cells (B cells, cytotoxic T cells, macrophages).

Cytotoxic T Cells (CD8+)

• Recognize antigens presented by class I MHC on infected or abnormal cells.

• Destroy target cells by releasing perforin and granzymes, inducing apoptosis.

B Cells and Antibody-Mediated Immunity

B Cell Activation and Clonal Selection

• B cells recognize specific antigens and, upon activation (often with helper T cell assistance), proliferate and differentiate into plasma cells and memory B cells.

• Plasma Cells: Produce and secrete antibodies specific to the antigen.

• Memory B Cells: Provide long-term immunity by responding rapidly to future exposures.

Antibody Actions

• Neutralize toxins and pathogens.

• Opsonize pathogens for enhanced phagocytosis.

• Activate complement system to destroy pathogens.

Immunological Memory

Immunological memory is the ability of the immune system to respond more rapidly and effectively to pathogens that have been encountered previously. Memory cells (both T and B) circulate in the body and enable a quicker and stronger response upon re-exposure to the same antigen.

Summary Table: Innate vs. Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Specificity	Nonspecific	Specific to particular antigens
Response Time	Immediate (minutes to hours)	Delayed (days to weeks for first exposure)
Memory	None	Develops after exposure
Major Components	Physical barriers, phagocytes, NK cells, complement, inflammation, fever	B and T lymphocytes, antibodies

Key Equations

• Bulk Flow (Capillary Exchange):

$\text{Net Filtration Pressure (NFP)} = (\text{Blood Hydrostatic Pressure} + \text{Interstitial Fluid Osmotic Pressure}) - (\text{Blood Colloid Osmotic Pressure} + \text{Interstitial Fluid Hydrostatic Pressure})$

• Example: At the arterial end of a capillary, if hydrostatic pressure is higher than osmotic pressure, fluid leaves the capillary (filtration). At the venous end, if osmotic pressure is higher, fluid re-enters (reabsorption).