Adaptive Immunity: Mechanisms and Components

Adaptive Immunity

Overview of Adaptive Immunity

Adaptive immunity is the body's highly specific defense mechanism against microbial infections. It involves the recognition of specific antigens and the generation of tailored immune responses, including the formation of immunological memory.

• Primary response: The initial immune reaction upon first exposure to an antigen, involving activation of specific immune cells.

• Secondary response: A more rapid and robust response upon subsequent exposure to the same antigen, mediated by memory cells.

Dual Nature of the Adaptive Immune System

Humoral and Cellular Immunity

The adaptive immune system consists of two main branches: humoral immunity and cellular immunity, each with distinct roles and cellular components.

• Humoral immunity: Mediated by antibodies produced by B cells; effective against extracellular pathogens and toxins.

• B cells: Develop in the red bone marrow and are responsible for antibody production.

• Cellular immunity: Mediated by T cells; effective against intracellular pathogens such as viruses and some bacteria.

• T cells: Mature in the thymus and recognize antigens presented by MHC molecules on cell surfaces.

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

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

Cytokines: Chemical Messengers of Immune Cells

Types and Functions of Cytokines

Cytokines are signaling proteins that mediate communication between immune cells, orchestrating the immune response.

• Interleukins (IL): Facilitate communication between leukocytes.

• Chemokines: Induce migration of leukocytes to sites of infection (chemotaxis).

• Interferons (INFs): INF-γ stimulates immune responses; other INFs protect cells from viral infection.

• Tumor necrosis factor (TNF): Promotes inflammation.

• Hematopoietic cytokines: Stimulate development of white blood cells.

• Cytokine storm: Overproduction of cytokines, leading to tissue damage.

Antigens and Antibodies

Antigens

An antigen (immunogen) is any substance that provokes a specific immune response. Antigens are typically proteins or large polysaccharides and contain specific regions called epitopes that are recognized by antibodies.

• Epitopes (antigenic determinants): Specific regions on antigens recognized by antibodies.

• Haptens: Small molecules that are antigenic only when attached to a carrier protein; can react with antibodies independently once formed.

Antibodies (Immunoglobulins)

Antibodies are Y-shaped proteins produced by B cells in response to antigens. Each antibody consists of two heavy chains and two light chains, forming a flexible structure with two antigen-binding sites.

• Variable (V) region: Contains the antigen-binding site; varies between antibodies.

• Constant (C) region: Determines the antibody class and mediates effector functions.

• Hinge region: Provides flexibility to the antibody molecule.

Class	Structure	Main Functions
IgG	Monomer	Most abundant in serum; crosses placenta; neutralizes toxins; activates complement; enhances phagocytosis
IgM	Pentamer	First antibody produced; agglutination; activates complement
IgA	Monomer (serum), Dimer (secretory)	Protects mucosal surfaces
IgD	Monomer	Functions as B cell receptor
IgE	Monomer	Involved in allergic reactions and defense against parasites

Humoral Immunity Response Process

B Cell Activation

B cells recognize antigens via surface antibodies and can be activated by two mechanisms:

• T-independent antigens: Repetitive epitopes directly activate B cells without T cell help.

• T-dependent antigens: Require antigen processing and presentation by B cells to helper T cells, which then provide signals for B cell activation.

Clonal Expansion and Differentiation

• Activated B cells differentiate into plasmocytes (antibody-secreting cells) and memory cells.

• Plasmocytes initially produce IgM, then switch to other classes (usually IgG).

• B cells recognizing self-antigens are eliminated by clonal deletion to prevent autoimmunity.

• Immunoglobulin gene rearrangement generates antibody diversity.

Results of Antigen–Antibody Interaction

Mechanisms of Action

• Antigen–antibody complex: Forms when an antibody binds to its specific epitope.

• Agglutination: Antibodies cross-link antigens, causing clumping of cells or particles.

• Opsonization: Coating of antigens with antibodies enhances phagocytosis.

• Neutralization: Antibodies block the activity of toxins or prevent pathogen entry into cells.

• Complement activation: Leads to lysis of bacterial cells.

Cellular Immunity Response Process

T Cell Maturation and Selection

• T cells mature in the thymus, where thymic selection eliminates self-reactive cells.

• T helper cells (CD4+) recognize antigens presented by MHC II on antigen-presenting cells (APCs).

• Cytotoxic T cells (CD8+) recognize antigens presented by MHC I on all nucleated cells.

Antigen-Presenting Cells (APCs)

• APCs include B cells, dendritic cells, and macrophages.

• Dendritic cells are crucial for activating naïve T helper cells.

• Activated macrophages are potent phagocytes and APCs.

• APCs transport antigens to lymphoid tissues for T cell recognition.

Classes of T Cells

• T helper (CD4+) cells: Differentiate into subtypes:

  • TH1: Involved in cellular and humoral immunity.

  • TH2: Involved in humoral immunity, allergic reactions, and defense against parasites.

  • TH17: Activate innate immunity.

• T regulatory (Treg) cells: Suppress immune responses against self-antigens.

• Cytotoxic T lymphocytes (CTLs, CD8+): Destroy infected or abnormal cells by inducing lysis or apoptosis.

Nonspecific Cells and Extracellular Killing

Natural Killer (NK) Cells and ADCC

• NK cells: Destroy virus-infected cells, tumor cells, and parasites lacking MHC I.

• Antibody-dependent cell-mediated cytotoxicity (ADCC): NK cells and macrophages lyse antibody-coated target cells.

Immunological Memory

Antibody Titer and Response Kinetics

• Antibody titer: The concentration of antibodies in serum.

• Primary response: Peak IgG titer occurs 10–17 days after antigen exposure.

• Secondary response: Peak titer occurs 2–7 days after re-exposure, with higher magnitude due to memory cells.

Types of Adaptive Immunity

Classification of Immunity

Type	How Acquired	Duration
Naturally acquired active	Infection	Long-lasting
Naturally acquired passive	Maternal antibodies (placenta, colostrum)	Few months
Artificially acquired active	Vaccination	Long-lasting
Artificially acquired passive	Injection of antibodies (antiserum/gamma globulin)	Few weeks

Example: Vaccination against measles induces artificially acquired active immunity, while administration of rabies immunoglobulin provides artificially acquired passive immunity.

Additional info: The complement system, which enhances antibody and phagocyte function, is closely linked to adaptive immunity and is often discussed alongside these topics.