Adaptive Immunity: Principles and Mechanisms

Adaptive Immunity

Overview of Adaptive Immune Response

The adaptive immune response is the body's third and final line of defense, activated when innate immunity fails to contain a threat. It is characterized by specificity to particular antigens and immunological memory, allowing for rapid and effective responses upon secondary exposure.

• Adaptive responses are acquired as the body encounters specific antigens and learns to remember them.

• Primary exposure takes days to weeks; secondary exposure is rapid and often prevents disease symptoms.

• The adaptive immune system is divided into two branches: Cellular response (T cell–mediated) and Humoral response (antibody–mediated).

• Both branches aim to eliminate identified antigens and remember them for future encounters.

Stages of Adaptive Immune Response

Adaptive immunity progresses through four main stages, involving both cellular and humoral responses.

• Stage 1: Antigen Presentation – Dendritic cells and other antigen-presenting cells (APCs) display antigens to T cells; B cells can directly interact with antigens.

• Stage 2: Lymphocyte Activation – Lymphocytes are activated by cytokines after antigen presentation; T cells influence B cell activation.

• Stage 3: Lymphocyte Proliferation and Differentiation – Activated B and T cells undergo clonal expansion, forming effector and memory cells.

• Stage 4: Antigen Elimination and Memory – Effector cells eliminate antigens; memory cells persist in lymphatic tissues for rapid response upon re-exposure.

Key Cells of Adaptive Immunity

T Cells and B Cells

Lymphocytes are the primary leukocytes involved in adaptive immunity. T cells mature in the thymus, while B cells mature in the bone marrow. Both cell types reside mainly in lymphoid tissues and are capable of recognizing a vast array of antigens.

• T cells participate in both humoral and cellular immunity.

• B cells coordinate the humoral response by producing antibodies.

Antigen Features and Immunogenicity

Antigen and Immunogenicity

An antigen is any substance that can trigger an immune response, typically proteins or polysaccharides from pathogens or cancer cells. Immunogenicity depends on antigen size, complexity, and composition.

• Complete antigens are highly immunogenic.

• Haptens are incomplete antigens, requiring linkage to larger molecules to trigger a response.

Epitopes

Epitopes are specific regions of an antigen recognized by B and T cells. Each lymphocyte is monospecific, recognizing only one epitope type.

Antigen Recognition Receptors

T cell receptors (TCRs) and B cell receptors (BCRs) are responsible for antigen recognition. Each cell has thousands of identical receptors, binding to the same epitope.

T Cell Subtypes and Functions

Helper and Cytotoxic T Cells

T cells are classified into two main types: helper (CD4+) and cytotoxic (CD8+). Helper T cells coordinate immune responses, while cytotoxic T cells directly destroy infected or cancerous cells.

• TH1 cells activate cytotoxic T cells, macrophages, and NK cells.

• TH2 cells stimulate B cells to produce antibodies.

• T regulatory cells control other WBC functions.

Self-Tolerance Screening

T and B Cell Screening

To prevent autoimmunity, T and B cells are screened for self-tolerance during maturation. T cells are tested for recognition of self-MHC proteins in the thymus, while B cells are screened in the bone marrow to ensure antibodies do not cross-react with self-antigens.

Antigen Presentation: MHC Molecules

MHC I and MHC II

Major histocompatibility complex (MHC) proteins are essential for antigen presentation. MHC I is found on all nucleated cells and presents intracellular antigens to cytotoxic T cells. MHC II is found only on APCs and presents extracellular antigens to helper T cells.

T Cell Activation

Activation Signals

T cells require two signals for activation: primary (TCR-MHC-antigen interaction) and secondary (co-stimulatory protein interaction). The type of signaling cascade determines the T cell subclass.

Superantigens

Superantigens are potent activators of T helper cells, causing broad, nonspecific activation and potentially dangerous cytokine release.

T Cell Proliferation and Differentiation

Clonal Expansion

Activated T cells undergo clonal expansion and differentiate into effector and memory cells. Cytokines released by APCs influence the development of T helper cell subclasses.

Antigen Elimination

T Cytotoxic Cell Mechanisms

T cytotoxic cells eliminate infected or cancerous cells by releasing perforins and granzymes, inducing apoptosis. Cytokines attract NK cells and macrophages to clear dead cells.

T Helper Cell Roles

T helper cells support the action of B cells, cytotoxic T cells, and innate leukocytes, coordinating the immune response.

B Cell Activation and Antibody Production

B Cell Activation Pathways

B cells can be activated by T-dependent or T-independent antigens. Most antigens require T helper cells for full activation, involving a two-signal mechanism.

B Cell Proliferation and Differentiation

Activated B cells proliferate and differentiate into plasma cells (effector cells) and memory B cells. Plasma cells secrete antibodies specific to the activating epitope.

Antibody Functions and Structure

Antibody Actions

Antibodies, or immunoglobulins (Ig), bind to antigens, neutralize them, activate complement cascades, and promote phagocytosis through precipitation, agglutination, and opsonization.

Antibody Structure

Antibodies are composed of two heavy and two light chains, held together by disulfide bonds. The variable region at the tips binds to specific epitopes, while the constant region forms the stem.

Antibody Isotypes

There are five main antibody isotypes, each with distinct functions and locations:

Isotype	Structure	Location	Function
IgG	Monomer	Blood, all fluids	Crosses placenta, activates complement, opsonin
IgA	Monomer/Dimer	Mucous membranes, secretions	Neutralization, opsonization
IgM	Monomer/Pentamer	Blood	Agglutination, complement activation
IgE	Monomer	Lungs, skin, mucous membranes	Allergy, parasite defense
IgD	Monomer	B cell surface	Unknown

Immunological Memory

Memory Cells

Memory B and T cells reside in lymphoid tissues, allowing for rapid and amplified responses upon re-exposure to the same antigen. Primary exposure produces IgM first, then IgG; secondary exposure rapidly produces high-affinity IgG.

Acquisition of Humoral Immunity

Types of Adaptive Immunity

Humoral immunity can be acquired naturally or artificially, and is classified as passive or active:

• Naturally acquired active immunity – Infection triggers immune response; long-term protection.

• Artificially acquired active immunity – Vaccination triggers immune response; long-term protection.

• Naturally acquired passive immunity – Antibodies received non-medically (e.g., maternal); temporary protection.

• Artificially acquired passive immunity – Antibodies received medically (e.g., antiserum); temporary protection.

Summary Table: T Cells vs. B Cells

Feature	T Cells	B Cells
Origin	Bone marrow, mature in thymus	Bone marrow
Main Function	Cellular immunity	Humoral immunity
Antigen Recognition	MHC-dependent	Direct or MHC II-dependent
Effector Cells	Cytotoxic, helper, regulatory	Plasma cells
Memory Cells	Yes	Yes

Key Concepts Illustrated

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