Lymphatic and Immune System: Study Guide Notes

Chapter 19 and 20: Lymphatic and Immune 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.

• Main Functions:

  • Return of fluid to lymphatic organs

  • Filtration of lymph through lymph nodes

  • Absorption of dietary fats (lacteals)

  • Immune surveillance and response

• Lymph Node: A small, bean-shaped structure that filters lymph and stores white blood cells to help fight infection.

• Lymphatic Capillaries: Tiny, thin-walled vessels located in the spaces between cells which serve to drain and process extracellular fluid.

• Lymphatic System Anatomy:

  • Lymphatic capillaries → collecting vessels → lymphatic trunks → lymphatic ducts

  • Lymphatic ducts drain into the subclavian veins

• Lacteal: Specialized lymphatic capillaries in the small intestine that absorb dietary fats.

General Structure of Lymphatic Tissue

Lymphatic tissue is primarily composed of reticular connective tissue, which provides a supportive framework for lymphocytes and other immune cells.

• Reticular Connective Tissue: A type of connective tissue with a network of reticular fibers that support lymphoid organs.

Lymph Node Anatomy

Lymph nodes filter lymph and house lymphocytes that respond to pathogens.

• Structure:

  • Multiple afferent vessels (bring lymph in)

  • Fewer efferent vessels (drain lymph out)

  • Germinal centers, capsule, and trabeculae

• Function: Filtration of lymph and activation of the immune response.

Tonsils and Thymus

• Tonsils: Lymphoid tissues located in the pharynx that protect against inhaled or ingested pathogens.

• Thymus: Site of T cell maturation; shrinks with age (involution).

Mucosa-Associated Lymphoid Tissue (MALT)

MALT includes lymphoid tissues associated with mucosal surfaces, such as Peyer's patches in the small intestine, which guard against pathogens entering through mucous membranes.

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

• Appendix: Contains lymphoid tissue that may help protect the gut.

Body Defenses: Innate and Adaptive Immunity

The body has two main lines of defense against pathogens: innate (nonspecific) and adaptive (specific) immunity.

• First Line of Defense: Physical and chemical barriers (skin, mucous membranes, secretions).

• Second Line of Defense: Internal defenses (phagocytes, inflammation, fever, antimicrobial proteins, natural killer cells).

• Third Line of Defense: Adaptive immunity (lymphocytes, antibodies).

Phagocytosis and Complement System

• Phagocytosis: The process by which phagocytes (e.g., neutrophils, macrophages) engulf and digest pathogens.

• Complement Proteins: Plasma proteins that enhance immune responses by promoting inflammation, opsonization, and cell lysis.

• Functions of Complement:

  1. Opsonization (coating pathogens to enhance phagocytosis)

  2. Agglutination of cells bearing foreign antigens

  3. Precipitation of soluble antigens

  4. Activation of the membrane attack complex (MAC) for cell lysis

  5. Neutralization of toxins or foreign matter

Cells of the Immune System

• Neutrophils: Most abundant phagocytes; first responders to infection.

• Macrophages: Large phagocytes derived from monocytes; present antigens to T cells.

• Natural Killer (NK) Cells: Destroy virus-infected and cancerous cells by releasing cytotoxic granules.

Adaptive (Specific) Immunity

Adaptive immunity involves lymphocytes (B and T cells) and is characterized by specificity and memory.

• Humoral Immunity: Mediated by B lymphocytes and antibodies.

• Cell-Mediated Immunity: Mediated by T lymphocytes; targets infected or abnormal cells.

Cytokines and Interferons

• Cytokines: Small proteins released by cells that affect the behavior of other cells; include interleukins and interferons.

• Interferons (IFN): Proteins that protect non-virally infected host cells by interfering with viral replication.

Antigen Presenting Cells (APCs)

• APCs: Cells that process and present antigens to T cells; include dendritic cells, macrophages, and B cells.

B and T Lymphocytes

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

• T Lymphocytes: Mature in thymus; include helper T cells (CD4+) and cytotoxic T cells (CD8+).

Activation and Function of B and T Cells

• B Cell Activation: Upon activation, B cells differentiate into plasma cells that secrete antibodies.

• Antibody Structure: Each antibody has two antigen-binding sites; IgM is a pentamer, IgA is a dimer.

• T Cell Activation: Requires antigen presentation on MHC molecules.

Major Histocompatibility Complex (MHC)

• MHC I: Found on all nucleated cells; presents endogenous antigens to CD8+ T cells.

• MHC II: Found only on APCs; presents exogenous antigens to CD4+ T cells.

Type	Active	MHC Binding	MHC Found
CD4	T helper	Only to MHC II	Only on APCs
CD8	T cytotoxic	Only to MHC I	Any nucleated cell

Antibody Classes

• IgM: First antibody produced; pentamer structure.

• IgA: Found in mucosal areas; dimer structure.

• IgD: Functions mainly as an antigen receptor on B cells.

• IgG: Most abundant; crosses placenta.

• IgE: Involved in allergic responses.

Types of Immunity

• Active Immunity: Body produces its own antibodies (natural: infection; artificial: vaccination).

• Passive Immunity: Antibodies are transferred from another source (natural: mother to fetus; artificial: injection of antibodies).

Type	Example
Natural Active	Infection (e.g., chickenpox from contagious individual)
Artificial Active	Vaccination (e.g., flu vaccine)
Natural Passive	Antibodies passing from mother to fetus via placenta
Artificial Passive	Antivenom for snake bite

Transplants and Immune Response

• Types of Transplants: Autograft, isograft, allograft, xenograft.

• Best Match: Autograft (from self) is least likely to be rejected.

Immune Disorders

• Autoimmune Disease: Immune system fails to distinguish self from nonself, attacking the body's own tissues.

• HIV: Primarily infects CD4+ T helper cells, leading to immunodeficiency.

Primary vs. Secondary Immune Response

• Primary Response: First exposure to antigen; slower and less robust.

• Secondary Response: Subsequent exposure; faster and stronger due to memory cells.

Key Equations and Concepts

• Antibody Structure: $\text{Antibody} = 2 \text{Heavy Chains} + 2 \text{Light Chains}$

• Opsonization: $\text{Pathogen} + \text{Antibody/Complement} \rightarrow \text{Enhanced Phagocytosis}$

Additional info: Some details, such as the structure of lymph nodes and the function of complement proteins, have been expanded for academic completeness.