Adaptive Immunity

Distinctive Attributes of Adaptive Immunity

Adaptive immunity is a highly specific defense mechanism that develops in response to exposure to pathogens. It is characterized by several unique features:

• Specificity: Targets specific antigens through highly specialized receptors.

• Inducibility: Activated only in response to specific antigens.

• Clonality: Generates clones of lymphocytes specific to the encountered antigen.

• Unresponsiveness to Self: Normally does not attack the body's own cells (self-tolerance).

• Memory: Retains information about previously encountered antigens for faster future responses.

Types of White Blood Cells in Adaptive Immunity

Adaptive immunity primarily involves two classes of lymphocytes:

• B lymphocytes (B cells): Responsible for humoral immunity; produce antibodies.

• T lymphocytes (T cells): Responsible for cell-mediated immunity; include helper, cytotoxic, and regulatory T cells.

Divisions of Adaptive Immunity

• Humoral Immunity: Mediated by B cells and antibodies; targets extracellular pathogens.

• Cell-Mediated Immunity: Mediated by T cells; targets intracellular pathogens and abnormal cells.

Lymphatic System: Organs and Flow

• Primary Organs: Bone marrow (site of B cell maturation), thymus (site of T cell maturation).

• Secondary Organs: Lymph nodes, spleen, tonsils, and other lymphoid tissues where immune responses are initiated.

• Lymph Flow: Lymph is a fluid that circulates through lymphatic vessels, carrying immune cells and antigens.

Role of Bone Marrow, Thymus, Lymph Nodes, and Other Tissues

• Bone Marrow: Hematopoiesis and B cell development.

• Thymus: T cell maturation and selection.

• Lymph Nodes: Filter lymph and provide sites for antigen presentation and lymphocyte activation.

• Other Tissues: Spleen filters blood; mucosa-associated lymphoid tissue (MALT) protects mucosal surfaces.

Antigens and Immune Responses

Characteristics of Antigens Stimulating Immune Responses

• Foreignness: Non-self molecules are more immunogenic.

• Size: Larger molecules are generally more immunogenic.

• Complexity: More complex molecules (proteins, polysaccharides) are better antigens.

• Accessibility: Epitopes must be accessible to immune cells.

Antigenic Determinants (Epitopes)

• Epitopes: Specific regions of an antigen recognized by immune receptors.

• Haptens: Small molecules that become antigenic when attached to larger carriers.

Antibody Structure and Classes

• Immunoglobulins (Ig): Antibodies produced by B cells; five main classes: IgG, IgM, IgA, IgE, IgD.

• Structure: Y-shaped molecules with variable (antigen-binding) and constant regions.

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

Development of Lymphocytes

• B cells: Mature in bone marrow; differentiate into plasma cells and memory B cells.

• T cells: Mature in thymus; differentiate into helper, cytotoxic, and regulatory T cells.

Immune Cell Activation and Response

Activation of T Cells

• Antigen Presentation: T cells require antigens to be presented by antigen-presenting cells (APCs) on major histocompatibility complex (MHC) molecules.

• Clonal Selection: Only T cells with receptors specific to the presented antigen are activated and proliferate.

Clonal Deletion and Editing

• Clonal Deletion: Removal of self-reactive lymphocytes during development to prevent autoimmunity.

• Clonal Editing: Further refinement of lymphocyte specificity.

Cytokines

• Cytokines: Signaling proteins that regulate immune responses, including interleukins, interferons, and tumor necrosis factors.

Antigen Presentation and MHC

Major Histocompatibility Complex (MHC) Proteins

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

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

Antigen-Presenting Cells (APCs)

• Dendritic Cells: Professional APCs; capture and present antigens to T cells.

• Macrophages: Engulf pathogens and present antigens.

• B Cells: Can present antigens to helper T cells.

Endogenous vs. Exogenous Antigen Processing

• Endogenous: Antigens from within the cell (e.g., viral proteins) are processed and presented on MHC I.

• Exogenous: Antigens from outside the cell (e.g., bacteria) are processed and presented on MHC II.

Immune Memory and Response Regulation

Memory T Cells

• Establishment: After initial activation, some T cells become memory cells for rapid response upon re-exposure.

Cell-Mediated Immunity

• Activation: Cytotoxic T cells destroy infected or abnormal cells.

• Helper T Cells: Coordinate immune responses by activating other immune cells.

Antibody-Mediated Immunity

• Activation: B cells differentiate into plasma cells that secrete antibodies.

• Memory B Cells: Provide long-term immunity.

Types of Immunity

Active vs. Passive Immunity

• Active Immunity: Results from direct exposure to antigen; produces memory cells.

• Passive Immunity: Results from transfer of antibodies (e.g., maternal antibodies); no memory cells produced.

Natural vs. Artificial Immunity

• Natural Immunity: Acquired through natural exposure to pathogens.

• Artificial Immunity: Acquired through medical intervention (e.g., vaccination).

Innate vs. Adaptive Immunity

• Innate Immunity: Non-specific, immediate defense mechanisms (e.g., barriers, phagocytes).

• Adaptive Immunity: Specific, slower to develop, but provides long-lasting protection.

Summary Table: Types of Immunity

Key Equations and Concepts

• Clonal Selection Theory: Each lymphocyte bears a single type of receptor with unique specificity.

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

Example: Upon exposure to influenza virus, memory B cells rapidly produce specific antibodies, providing immunity against future infections.

Additional info: Some details, such as the specific steps of T cell activation and the structure of immunoglobulins, were expanded for clarity and completeness.