Chapter 20: The Lymphatic and Immune Systems – Structure and Function

Section 20.1: The Lymphatic System

Lymphatic Functions

The lymphatic system is a vital component of the circulatory and immune systems, responsible for maintaining fluid balance, defending against pathogens, and facilitating the absorption of dietary fats.

• Regulation of Interstitial Fluid Volume: The lymphatic system collects excess interstitial fluid from tissues and returns it to the bloodstream, preventing edema.

• Immune Functions: Lymphatic tissues and organs house immune cells (lymphocytes, macrophages) that monitor and respond to pathogens.

• Absorption of Dietary Fats: Specialized lymphatic vessels called lacteals in the small intestine absorb dietary lipids and fat-soluble vitamins, transporting them to the bloodstream.

Fluid Compartments in the Body:

• Intracellular Fluid: Fluid within cells (about 25 L in adults).

• Extracellular Fluid: Fluid outside cells, including interstitial fluid (12 L) and plasma (3 L).

• Lymph: Interstitial fluid that enters lymphatic vessels.

Example: Lymphatic capillaries surrounding blood capillaries collect excess fluid and return it to the venous circulation, maintaining homeostasis.

Lymphatic Vessel Anatomy

Structure of Lymphatic Vessels

Lymphatic vessels are a network of tubes that transport lymph throughout the body. Their unique structure allows for efficient fluid collection and immune surveillance.

• Lymphatic Capillaries: Small, blind-ended vessels with overlapping endothelial cells that act as one-way flaps, opening in response to increased tissue pressure.

• Pressure Regulation: Flaps open and close with changes in interstitial pressure, allowing fluid entry but preventing backflow.

• Vessel Hierarchy: Lymphatic capillaries merge to form larger vessels, which drain into lymphatic trunks and ultimately into ducts (thoracic duct, right lymphatic duct).

Additional info: Lymphatic vessels are absent in avascular tissues such as cartilage and the cornea.

Lymphatic Tissue and Organs

Lymphatic Tissue Types

Lymphatic tissues are specialized sites for immune cell development and activation. They are classified based on their location and function.

• Mucosa-Associated Lymphatic Tissue (MALT): Clusters of lymphatic tissue found in mucous membranes of the digestive, respiratory, and genitourinary tracts. They protect against pathogens entering through mucosal surfaces.

• Lymph Nodes: Small, bean-shaped structures located along lymphatic vessels. They filter lymph and house lymphocytes and macrophages.

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

• Tonsils: Aggregates of lymphatic tissue in the pharynx that protect against inhaled or ingested pathogens.

Example: Submandibular, cervical, axillary, and inguinal lymph nodes are key sites for lymph filtration and immune activation.

Immune System Organization

Innate and Adaptive Immunity

The immune system is organized into two main branches: innate (nonspecific) and adaptive (specific) immunity. Both work together to protect the body from infection.

• Innate Immunity: Provides immediate, nonspecific defense against pathogens. Includes physical barriers (skin, mucous membranes), phagocytic cells, natural killer cells, and antimicrobial proteins.

• Adaptive Immunity: Develops more slowly and provides specific responses to pathogens. Involves lymphocytes (B cells and T cells) and the production of antibodies.

Additional info: Adaptive immunity has memory, allowing for faster and stronger responses upon re-exposure to the same pathogen.

Antigen Recognition and Immune Response

Antigens and Immunogens

Antigens are molecules capable of triggering an immune response. Immunogens are antigens that specifically elicit an adaptive immune response.

• Antigenic Determinants (Epitopes): Specific regions of an antigen recognized by antibodies or T cell receptors.

• B Cells: Have membrane-bound antibodies (immunoglobulins) that recognize free antigens.

• T Cells: Have receptors that recognize antigens only when presented by major histocompatibility complex (MHC) molecules.

Example: Pathogens display foreign antigens, while self-cells display self-antigens to prevent autoimmunity.

Major Histocompatibility Complex (MHC) and Antigen Presentation

MHC Molecules and T Cell Activation

MHC molecules are essential for antigen presentation to T cells, enabling the immune system to distinguish self from non-self.

• MHC I: Found on all nucleated cells; presents endogenous antigens (from within the cell) to cytotoxic T cells (CD8+).

• MHC II: Found on antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells; presents exogenous antigens (from outside the cell) to helper T cells (CD4+).

• T Cell Receptors: Bind to antigens only when displayed by MHC molecules.

Additional info: MHC I signals "help, I'm sick" for infected or cancerous cells, while MHC II signals "I've been invaded" for immune activation.

Effector Cells and Immune Functions

Effector T Cells

Effector T cells carry out immune responses by targeting infected or abnormal cells.

• Cytotoxic T Cells (CD8+): Destroy infected or cancerous cells by releasing perforins and granzymes.

• Helper T Cells (CD4+): Activate other immune cells by releasing cytokines.

Additional info: Memory T cells persist after infection, providing long-term immunity.

Antibodies and Humoral Immunity

Antibody Structure and Function

Antibodies (immunoglobulins) are proteins produced by B cells that recognize and neutralize antigens.

• IgG: Most abundant in serum; crosses placenta to provide passive immunity to fetus.

• IgA: Present in secretions (saliva, tears, mucous); protects mucosal surfaces.

• IgM: Largest antibody; first produced in response to infection; can bind 10 antigens.

• IgE: Binds to allergens and parasitic worms; involved in allergic reactions.

• IgD: Located on B cell surface; acts as B cell receptor.

Antibody Class	Main Function	Location
IgG	Most numerous; crosses placenta	Serum
IgA	Protects mucosal surfaces	Secretions
IgM	First response; binds 10 antigens	Serum, B cell surface
IgE	Allergic reactions; parasite defense	Mast cells, serum
IgD	B cell receptor	B cell surface

Additional info: Antibodies neutralize toxins, opsonize pathogens, and activate complement.

Immunological Memory

Primary and Secondary Immune Responses

Immunological memory allows the immune system to respond more rapidly and effectively upon subsequent exposures to the same antigen.

• Primary Response: Occurs after first exposure; slower and less robust.

• Secondary Response: Occurs after subsequent exposures; faster and stronger due to memory cells.

Example: Vaccination induces immunological memory, providing long-term protection.

Immune System Disorders

Types of Hypersensitivity Reactions

Hypersensitivity reactions are inappropriate or exaggerated immune responses to antigens.

• Type I (Immediate): Rapid allergic reactions (e.g., anaphylaxis).

• Type II (Antibody-Mediated): Cytotoxic reactions (e.g., ABO blood incompatibility).

• Type III (Immune Complex-Mediated): Deposition of antigen-antibody complexes (e.g., serum sickness).

• Type IV (Delayed-Type): T cell-mediated reactions (e.g., contact dermatitis).

Additional info: Autoimmune diseases result from immune responses against self-antigens.

Transplantation and Rejection

Types of Grafts

Transplantation involves transferring cells, tissues, or organs from one individual to another. Immune responses can lead to rejection.

• Autograft: Transplant from the same individual (no rejection).

• Allograft: Transplant between genetically different individuals of the same species (risk of rejection).

• Xenograft: Transplant between individuals of different species (high risk of rejection).

Additional info: Immunosuppressive drugs are used to prevent graft rejection.

The Complement System

Antimicrobial Proteins

The complement system consists of plasma proteins that enhance immune responses and help eliminate pathogens.

• Activation: Complement proteins are produced mainly by the liver and activated in response to infection.

• Functions: Enhance inflammation, neutralize viruses, promote phagocytosis, and lyse pathogens.

Additional info: Complement activation can occur via classical, alternative, or lectin pathways.

Cytokines and Inflammatory Response

Role of Cytokines

Cytokines are signaling molecules released by immune cells to coordinate the inflammatory response and recruit other cells to sites of infection.

• Mediators: Include interleukins, interferons, and tumor necrosis factor.

• Effects: Trigger fever, increase vascular permeability, and attract leukocytes.

Example: Pyrogens released during infection stimulate the hypothalamus to raise the body's set point, causing fever.

Summary Table: Key Components of the Lymphatic and Immune Systems

Component	Main Function	Example
Lymphatic Vessels	Return interstitial fluid to blood	Lacteals absorb dietary fats
Lymph Nodes	Filter lymph, house immune cells	Cervical, axillary nodes
Spleen	Filter blood, remove old RBCs	White pulp (lymphocytes)
B Cells	Produce antibodies	Plasma cells
T Cells	Cell-mediated immunity	Cytotoxic T cells
Antibodies	Neutralize antigens	IgG, IgA, IgM, IgE, IgD
Complement	Enhance immune response	Classical pathway
Cytokines	Coordinate inflammation	Interleukins