Adaptive Immunity: Specific Defense System in Human Physiology

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Specific Defense System: Adaptive Immunity

Overview of Adaptive Immunity

The adaptive immune system is a highly specialized defense mechanism that eliminates pathogens and abnormal cells through targeted responses. Unlike innate immunity, adaptive immunity requires priming by exposure to specific antigens, resulting in a delayed but highly effective response. Its main characteristics include specificity, systemic action, versatility, tolerance, and memory.

Specificity: Recognizes and targets specific antigens.
Systemic: Not restricted to the initial site of infection.
Versatility: Large diversity of lymphocytes.
Tolerance: Recognition of self-antigens to prevent autoimmunity.
Memory: Enhanced response upon subsequent exposures to the same antigen.

Main Branches of Adaptive Immunity

Humoral Immunity (Antibody-Mediated): Involves B lymphocytes and the production of antibodies.
Cellular Immunity (Cell-Mediated): Involves T lymphocytes, including helper T cells (TH) and cytotoxic T cells (TC).

Diagram of adaptive immunity branches and cytokine signaling

Antigens and Antigenic Determinants

Definition and Properties

Antigens are substances that provoke an immune response and mobilize adaptive defenses. Most antigens are large, complex molecules with multiple antigenic determinants (epitopes), which are specific regions recognized by antibodies, B cell receptors, or T cell receptors.

Immunogenicity: Ability to provoke an immune response.
Antigenic Determinants (Epitopes): Specific parts of an antigen that bind to immune receptors.
Complexity: Chemically simple molecules have low antigenicity; complex molecules (proteins, polysaccharides) have high antigenicity.

Antigenic determinants and antibody binding sites Epitopes on an antigen Antigen with different antigenic determinants Antibodies binding to antigenic determinant sites

Major Histocompatibility Complex (MHC) Proteins and Antigen Presentation

MHC Class I and II Proteins

MHC proteins are essential for antigen presentation and activation of T cells. There are two main classes:

MHC Class I: Displayed by all nucleated cells except RBCs; present endogenous antigens (self or non-self) to CD8 T cells (cytotoxic T cells).
MHC Class II: Displayed by antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells; present exogenous antigens to CD4 T cells (helper T cells).

MHC I antigen presentation pathway MHC I presenting foreign antigen MHC I presenting self antigen Dendritic cell presenting antigen Macrophage presenting antigen MHC II antigen presentation pathway

Cytokines: Chemical Messengers of Immunity

Role and Types of Cytokines

Cytokines are proteins or peptides that regulate the intensity and duration of immune responses by binding to specific cell surface receptors. They stimulate proliferation and differentiation of immune cells and include interleukins, interferons, and lymphokines.

Interleukin-1 (IL-1): Secreted by macrophages; promotes TH cell proliferation and induces fever.
Interleukin-2 (IL-2): Secreted by TH cells; co-stimulates proliferation of TH, TC, and B cells; activates NK cells.
Interleukin-3 (IL-3): Secreted by TH cells; stimulates macrophages and enhances phagocytosis.
Interferons (IFNs): IFN-α and IFN-β protect uninfected cells and stimulate NK cells; IFN-γ stimulates macrophages.
Interleukin-6 (IL-6): Secreted by TH cells; enhances B cell proliferation and differentiation into plasma cells.

Cytokine signaling pathways in immunity Interleukin-2 and Interleukin-6

T Lymphocyte Activation, Differentiation, and Proliferation

Mechanisms of T Cell Activation

T cell activation requires double recognition: the T cell receptor (TCR) must recognize both the MHC molecule and the antigen. Activation is a three-step process involving antigen recognition, co-stimulation, and clonal expansion.

Antigen Recognition: TCR binds to MHC-antigen complex on APC.
Co-stimulation: Involves cytokines (e.g., IL-1) and cell surface proteins (CD4, CD8, CD28, B7).
Clonal Expansion: Differentiation and proliferation of T lymphocytes, producing activated and memory TH and TC cells.

Activated TH cell and clonal expansion

Humoral Immune Response: B Lymphocyte Activation

B Cell Activation and Differentiation

B cells are activated when their B cell receptor (BCR) binds to a specific epitope on an antigen. This triggers receptor-mediated endocytosis and presentation to a helper T cell, leading to clonal expansion and differentiation into plasma cells and memory B cells.

Plasma Cells: Secrete antibodies at a high rate for several days.
Memory B Cells: Provide immunological memory for rapid response upon re-exposure.

B cell activation and differentiation

Immunological Memory

Immunological memory is established by memory B cells. The primary immune response occurs upon first exposure, with a lag period and peak antibody levels after about 10 days. The secondary response is faster, stronger, and longer-lasting due to sensitized memory B cells.

Antibodies (Immunoglobulins)

Structure and Classes

Antibodies are Y-shaped proteins composed of two heavy and two light chains, with variable regions forming antigen-binding sites. The constant region determines the antibody class and its function.

Classes: IgG, IgA, IgM, IgE, IgD
Functions: Complement fixation, circulation in blood, presence in body secretions

Antibody Targets and Functions

Defensive Mechanisms: Precipitation, lysis (complement fixation), agglutination, neutralization, degranulation
Enhances: Phagocytosis (opsonization), inflammation, chemotaxis

Cellular Immune Response: T Cell Defense

Cytotoxic T Cell (TC) Function

Cytotoxic T cells directly kill cells infected with viruses, bacteria, cancerous or abnormal cells, and foreign cells. They release perforins and granzymes to induce apoptosis in target cells.

Activation: Requires antigen presentation by MHC I and co-stimulation by helper T cells.
Targets: Virus-infected cells, cells with intracellular bacteria or parasites, cancer cells, foreign cells.

Immunological Pathologies

Immunodeficiencies

SCID: Genetic disease affecting T, B, and NK cells; treated with bone marrow transplants.
AIDS: Caused by HIV, which impairs TH cell function, leading to opportunistic infections.

Regulatory T Lymphocytes (Tregs)

Tregs maintain immune homeostasis and self-tolerance, preventing autoimmunity and excessive immune responses. They are specialized CD4 T cells and can be natural (nTregs) or induced (iTregs).

Autoimmune Diseases

Examples: Rheumatoid arthritis, myasthenia gravis, multiple sclerosis, Graves’ disease, type 1 diabetes mellitus, systemic lupus erythematosus (SLE)
Treatment: Suppression of the immune system with anti-inflammatory drugs

Hypersensitivities

Hypersensitivity reactions are excessive adaptive immune responses to harmless antigens. Type I hypersensitivity (allergies) is IgE-mediated and results in rapid inflammatory responses upon exposure to allergens.

Systemic response: Anaphylactic shock, treated with epinephrine
Local response: Involves mast cells in skin or mucosa; treated with antihistamines

Summary Table: Types of Adaptive Immune Cells and Their Response Time

Cell Type	Function	Response Time
Helper T Cell (TH)	Activates B cells, TC cells, macrophages	Primary response peaks in 1 week
Cytotoxic T Cell (TC)	Kills infected, abnormal, or foreign cells	Immediate upon activation
B Cell	Produces antibodies (plasma cells), memory cells	Primary: 7-14 days; Secondary: 1-3 days

Additional info: This summary integrates textbook-level explanations and visual aids to reinforce key concepts in adaptive immunity, suitable for exam preparation in anatomy and physiology courses.