Adaptive Immunity and Immune System Disorders: ANP Study Guide

Adaptive Immunity

Overview of Adaptive Immunity

Adaptive immunity is a specific defense mechanism that targets particular pathogens and provides long-lasting protection through memory. It is divided into humoral and cellular immunity, each with distinct roles and mechanisms.

• Specificity: Adaptive immunity is directed against specific pathogens, allowing precise targeting.

• Memory: Upon re-exposure to the same pathogen, the immune response is rapid and effective, often preventing illness.

Humoral Immunity

Definition and Types

Humoral immunity involves antibody-mediated responses, primarily carried out by B lymphocytes. It can be acquired actively or passively, either naturally or artificially.

• Active immunity: Results from direct exposure to antigens, leading to antibody production.

• Passive immunity: Involves the transfer of antibodies from another source.

Antigens and Epitopes

Antigens are substances that trigger an immune response by prompting the generation of antibodies. The immune system recognizes specific regions of antigens called epitopes or antigenic determinants.

• Antigen: Any molecule capable of inducing an immune response.

• Epitope: The specific part of an antigen recognized by antibodies; antigens often have multiple epitopes.

• Complexity: More complex molecules (e.g., proteins) are more antigenic than simpler ones (e.g., lipids).

B Lymphocytes (B Cells)

B cells are responsible for humoral immunity and develop in the red bone marrow. They undergo selection to ensure they do not react to self-antigens, and those that do are destroyed.

• Development: Occurs in the red bone marrow.

• Selection: B cells reactive to self-antigens are eliminated.

• Colonization: Mature B cells colonize lymph organs.

• Activation: When a B cell encounters its specific antigen, it becomes activated and differentiates into plasma cells and memory B cells.

Antibody Structure

Antibodies, also known as immunoglobulins (Ig), are defensive proteins found in blood plasma and body secretions. They consist of four polypeptide chains: two heavy and two light chains.

• Variable (V) regions: Bind to specific antigens.

• Constant (C) regions: Determine antibody class and function.

• Disulfide bonds: Stabilize the structure.

Antibody Classes

Antibodies are classified based on their constant regions, each with unique functions and structures.

• IgG: Monomer; most abundant, crosses placenta, secondary immune response.

• IgD: Monomer; B cell membrane receptor.

• IgE: Monomer; on mast cells, triggers histamine release.

• IgA: Monomer in plasma, dimer in secretions; prevents adherence to epithelia.

• IgM: Pentamer; primary immune response.

Humoral Immunity Responses

The humoral immune response involves the activation of B cells, production of plasma cells, and formation of memory B cells. The secondary response is faster and more robust due to memory cells.

• Primary response: Initial exposure to antigen activates B cells and produces antibodies.

• Secondary response: Subsequent exposure leads to rapid antibody production and prevents illness.

Cellular Immunity

Definition and T Cell Classes

Cellular immunity is mediated by T lymphocytes, which attack foreign cells and diseased host cells. There are three main classes of T cells:

• Cytotoxic T cells (Tc, CD8): Directly attack infected or abnormal cells.

• Helper T cells (Th, CD4): Promote activation of Tc and B cells, coordinate immune responses.

• Memory T cells (Tm): Provide long-term immunity by launching quick attacks upon re-exposure.

Life Cycle of T Cells

T cells originate from stem cells in the red bone marrow and mature in the thymus. They colonize lymphatic tissues and organs, and self-reactive cells are destroyed.

• Origin: Stem cells in red bone marrow.

• Maturation: Occurs in the thymus.

• Colonization: Lymphatic tissues and organs.

• Selection: Self-reactive T cells are eliminated.

Antigen Processing and Presentation

Antigen-presenting cells (APCs), such as B cells and macrophages, phagocytize antigens and present fragments on their surface with MHC proteins.

• Phagocytosis: Antigen is engulfed by APC.

• Lysosome fusion: Antigen is degraded.

• Presentation: Fragments are displayed with MHC (Major Histocompatibility Complex) proteins.

• MHC I: Present on all nucleated cells; recognized by CD8 T cells.

• MHC II: Present on APCs; recognized by CD4 T cells.

T Cell Activation

T cells are activated when they recognize antigens presented by APCs. This leads to proliferation of T cell clones and memory cells.

• Tc cells: Directly attack pathogens.

• Th cells: Secrete interleukins, attract and stimulate other immune cells, coordinate cellular and humoral immunity.

Immune System Disorders

Hypersensitivity (Allergy)

Hypersensitivity is an exaggerated immune response to harmless antigens, leading to allergic reactions.

• Anaphylactic shock: Acute, severe allergic reaction causing bronchiole constriction, dyspnea, vasodilation, shock, and potentially death. Treatment includes epinephrine.

Autoimmune Disease

Autoimmune diseases occur when the immune system fails to recognize self-antigens and attacks the body's own tissues.

• Mechanisms: Cross-reactivity, structural changes in self-antigens, viral or drug-induced changes.

• Examples: Type I diabetes mellitus, lupus, rheumatoid arthritis.

AIDS: Human Immunodeficiency Virus (HIV)

HIV is a virus that primarily invades helper T cells (CD4), leading to acquired immunodeficiency syndrome (AIDS).

• Replication: Viruses replicate only within host cells.

• Impact: Depletion of CD4 cells (<200 cells/μL) increases susceptibility to infections.