Adaptive Immunity: The Specific Defense System

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Adaptive Immunity: Overview

Introduction to Adaptive Immunity

The adaptive (specific) immune system is a highly specialized defense mechanism that eliminates almost any pathogen or abnormal cell in the body. Unlike the innate immune system, adaptive immunity must be primed by initial exposure to a specific foreign substance, a process known as priming, which takes time.

Specificity: Recognizes and targets specific antigens.
Systemic Response: Not restricted to the initial site of infection.
Memory: Mounts a stronger attack upon subsequent exposures to the same antigen.

Main Branches of Adaptive Immunity

Humoral Immunity (Antibody-Mediated): Involves B lymphocytes and the production of antibodies that circulate in body fluids.
Cell-Mediated Immunity: Involves T lymphocytes that directly attack infected or abnormal cells.

Cells of Adaptive Immunity

Lymphocytes

Lymphocytes are the primary cells of adaptive immunity, consisting of two main types:

B Lymphocytes (B cells): Responsible for humoral immunity; differentiate into plasma cells that secrete antibodies.
T Lymphocytes (T cells): Responsible for cell-mediated immunity; include several subtypes:
- Helper T Cells (Th): Activate other lymphocytes, including B cells and cytotoxic T cells, and stimulate macrophages. They enhance the immune response by releasing cytokines.
- Cytotoxic T Cells (Tc): Directly kill virus-infected cells and tumor cells.

Cytokines

Cytokines are small peptides that act as signaling molecules in the immune system. They regulate the intensity and duration of immune responses and promote the proliferation and differentiation of immune cells.

Examples include interferons (IFNs), interleukins (ILs), and lymphokines.

Antigens and Antigenic Determinants

Definition of Antigen

An antigen (Ag) is any substance that provokes an immune response and mobilizes adaptive defenses. Most antigens are large, complex molecules (molecular weight > 10,000).

Antigenic Determinants (Epitopes)

Antigenic determinant (epitope): The specific part of an antigen that a specific antibody, B cell receptor, or T cell receptor recognizes and binds to.
Most naturally occurring antigens have several antigenic determinants, allowing multiple lymphocyte populations to recognize them.
Large, complex molecules (e.g., proteins, polysaccharides) have greater antigenicity than small, simple molecules (e.g., plastics).

Self-Antigens

Self-antigens are molecules on the surface of an individual's own cells. The immune system typically recognizes these as 'self' and does not attack them under normal conditions.

Major Histocompatibility Complex (MHC) Proteins and Antigen Presentation

MHC Proteins

MHC proteins are cell surface proteins essential for the presentation of antigens to T cells. There are two main classes:

Class I MHC: Found on all nucleated cells; present endogenous antigens (from within the cell) to cytotoxic T cells (CD8+).
Class II MHC: Found on antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells; present exogenous antigens (from outside the cell) to helper T cells (CD4+).

Antigen Presentation Process

APCs process antigens and display them on their surface bound to MHC proteins.
T cells recognize antigens only when presented by MHC molecules.

Activation and Differentiation of Lymphocytes

B Cell Activation

B cells encounter their specific antigen and become activated.
Activated B cells differentiate into plasma cells (which secrete antibodies) and memory B cells (which provide long-term immunity).

T Cell Activation

T cells require antigen presentation by MHC proteins on APCs.
Helper T cells (CD4+) are activated by antigens presented on Class II MHC.
Cytotoxic T cells (CD8+) are activated by antigens presented on Class I MHC.
Activated T cells proliferate and differentiate into effector and memory cells.

Immunological Memory

Primary and Secondary Immune Responses

Primary response: Occurs after initial exposure to an antigen; slower and less robust.
Secondary response: Occurs upon subsequent exposures; faster and more effective due to memory cells.

Antibodies (Immunoglobulins)

Structure and Function

Antibodies are Y-shaped proteins composed of two identical heavy (H) chains and two identical light (L) chains.
The hinge region allows flexibility for binding antigens.
Antibodies bind to antigenic determinants and neutralize, agglutinate, or precipitate antigens.

Immune Pathologies

Immunodeficiency

Severe Combined Immunodeficiency (SCID): Genetic disorder resulting in defective or absent immune responses.
Acquired Immunodeficiency Syndrome (AIDS): Caused by HIV infection, leading to the destruction of helper T cells.

Autoimmune Diseases

Occur when the immune system attacks self-antigens.
Examples include rheumatoid arthritis, myasthenia gravis, multiple sclerosis, Graves' disease, type 1 diabetes mellitus, and systemic lupus erythematosus (SLE).

Hypersensitivity Reactions (Allergies)

Type I (Immediate) Hypersensitivity: Rapid allergic reaction mediated by IgE antibodies and mast cells.
Type IV (Delayed) Hypersensitivity: T cell-mediated response occurring hours to days after exposure.
Systemic reactions can lead to anaphylactic shock, requiring immediate treatment (e.g., epinephrine).

Table: Comparison of Humoral and Cell-Mediated Immunity

Feature	Humoral Immunity	Cell-Mediated Immunity
Main Cell Type	B lymphocytes	T lymphocytes
Effector Molecule	Antibodies	Cytokines, cytotoxic T cells
Target	Extracellular pathogens (bacteria, viruses in body fluids)	Intracellular pathogens (virus-infected cells, tumor cells)
Antigen Presentation Required	No (direct recognition)	Yes (via MHC proteins)
Memory Formation	Yes	Yes

Key Equations

Antibody-Antigen Binding:

Immune Response Kinetics:

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