The Immune System

Main Divisions of the Immune System

The immune system is divided into two main branches: the innate (nonspecific) immunity and the adaptive (specific) immunity. Each plays a crucial role in defending the body against pathogens.

• Innate Immunity: The first line of defense, providing immediate but non-specific protection.

• Adaptive Immunity: Develops more slowly and provides specific responses to particular pathogens, including memory for future responses.

Lymphatic System Overview

The lymphatic system is a network of vessels, nodes, and organs that helps maintain fluid balance, absorbs fats, and contributes to immune defense.

• Main Functions: Returns interstitial fluid to the bloodstream, absorbs and transports fats, and provides immune responses.

• Main Organs: Lymph nodes, spleen, thymus, tonsils, and lymphatic vessels.

Organization of Blood Vessels and Capillaries

Blood and lymphatic capillaries are organized to drain excess fluid from tissues. Lymphatic capillaries collect lymph and transport it through lymphatic vessels, eventually draining into the thoracic duct or right lymphatic duct.

• Thoracic Duct: Drains lymph from most of the body into the left subclavian vein.

• Right Lymphatic Duct: Drains lymph from the right upper limb, right side of thorax, and right halves of head and neck into the right subclavian vein.

Lymphatic Organs and Tissues

Lymphatic organs include primary organs (bone marrow and thymus) where lymphocytes mature, and secondary organs (lymph nodes, spleen, tonsils) where immune responses are initiated.

• Lymph Nodes: Filter lymph and house lymphocytes.

• Spleen: Filters blood, removes old red blood cells, and mounts immune responses.

• Thymus: Site of T cell maturation.

Lines of Defense

Three Lines of Defense

The body defends itself through three main lines of defense:

1. Physical and Chemical Barriers: Skin, mucous membranes, secretions.

2. Innate Immune Cells: Phagocytes (macrophages, neutrophils), natural killer cells, inflammation, fever.

3. Adaptive Immunity: Lymphocytes (B cells and T cells), antibodies.

Cells of the Immune System

• Macrophages: Engulf and digest pathogens.

• Neutrophils: First responders to infection, phagocytic.

• Eosinophils, Basophils: Involved in allergic responses and parasitic infections.

• Dendritic Cells: Antigen-presenting cells that activate T cells.

Inflammation and Cytokines

Inflammation is a protective response involving immune cells, blood vessels, and molecular mediators. Cytokines and interleukins are signaling molecules that regulate immune responses.

Adaptive Immunity

Definition and Types

Adaptive immunity is the body's ability to recognize and remember specific pathogens and mount stronger attacks each time the pathogen is encountered. It involves two main types of responses:

• Cell-mediated Immunity: Involves T cells (Helper T cells - CD4+, Cytotoxic T cells - CD8+).

• Antibody-mediated (Humoral) Immunity: Involves B cells and the production of antibodies.

T Cells

• Helper T Cells (CD4+): Activate B cells and other immune cells.

• Cytotoxic T Cells (CD8+): Destroy infected or abnormal cells.

• Activation: Requires antigen presentation by MHC molecules.

B Cells

• Plasma Cells: Produce antibodies.

• Memory B Cells: Provide long-term immunity.

• Maturation: Occurs in bone marrow.

Major Histocompatibility Complex (MHC)

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

• MHC II: Present on antigen-presenting cells; present exogenous antigens to CD4+ T cells.

Antibodies and Antigens

Antibody Structure and Function

Antibodies (immunoglobulins) are Y-shaped proteins produced by B cells that bind to specific antigens.

• Structure: Two heavy chains and two light chains, with variable regions for antigen binding.

• Epitope: The specific part of the antigen recognized by the antibody.

• Classes: IgG, IgM, IgA, IgE, IgD.

Example: IgG is the most abundant antibody in blood and provides long-term protection.

Immunoglobulin Classes and Functions

• IgG: Main antibody in secondary response; crosses placenta.

• IgM: First antibody produced in primary response; pentameric structure.

• IgA: Found in mucosal areas and secretions.

• IgE: Involved in allergic responses.

• IgD: Functions mainly as a B cell receptor.

Immune Responses

Primary vs. Secondary Immune Response

• Primary Response: First exposure to antigen; slower, mainly IgM production.

• Secondary Response: Subsequent exposures; faster and stronger, mainly IgG production due to memory cells.

Clonal Selection and Memory

Clonal selection is the process by which lymphocytes that recognize a specific antigen are activated and proliferate. Memory cells ensure a rapid response upon re-exposure to the same antigen.

Cellular and Humoral Immunity

Cellular Immunity

• Cytotoxic T Cells: Destroy infected cells by inducing apoptosis.

• Helper T Cells: Activate other immune cells via cytokines.

Humoral Immunity

• B Cells: Recognize antigens, differentiate into plasma cells, and secrete antibodies.

• Antibody Functions: Neutralization, opsonization, complement activation.

Immunity Types and Disorders

Active vs. Passive Immunity

• Active Immunity: Results from exposure to antigen (infection or vaccination); long-lasting.

• Passive Immunity: Transfer of antibodies (e.g., maternal IgG to fetus, antiserum); temporary protection.

Example: Vaccines provide active immunity; immunoglobulin injections provide passive immunity.

Autoimmune Diseases and Allergies

• Autoimmune Disease: The immune system attacks self-tissues (e.g., rheumatoid arthritis, type 1 diabetes).

• Allergies: Hypersensitive immune responses to harmless antigens (allergens).

HIV and Immune Deficiency

• HIV: Human Immunodeficiency Virus infects CD4+ T cells, leading to immune deficiency (AIDS).

• Effect: Increased susceptibility to infections and certain cancers.

Key Table: Comparison of Innate and Adaptive Immunity

Key Equations and Concepts

• Antibody-Antigen Binding: The strength of binding is called affinity.

• Clonal Expansion: (where is the number of cells after divisions, is the initial number).

Additional info:

• Some details, such as the structure of lymph nodes and the process of antigen presentation, were expanded for clarity and completeness.

• Table content and equations were inferred and added for academic context.