Adaptive Immunity: T Cells, B Cells, and Antibody-Mediated Responses

Adaptive Immunity: Overview

Adaptive immunity is a highly specific defense mechanism that involves the recognition and elimination of foreign antigens by specialized lymphocytes. The two main branches are the cellular response (mediated by T cells) and the humoral response (mediated by B cells and antibodies).

T and B Lymphocytes

Types and Development

• T cells (CD8+ cytotoxic and CD4+ helper): Produced in the bone marrow, mature in the thymus.

• B cells: Produced and mature in the bone marrow.

• Both coordinate adaptive immune responses against foreign antigens.

Antigen Receptors

• T cell receptors (TCRs) and B cell receptors (BCRs) are specialized proteins that recognize specific epitopes on antigens.

• Each lymphocyte expresses thousands of identical receptors, but each cell recognizes only one epitope type.

• The diversity of T and B cells allows the immune system to recognize virtually any antigen.

Stages of the Adaptive Immune Response

The adaptive immune response proceeds through four main stages:

1. Antigen Presentation

2. Lymphocyte Activation

3. Lymphocyte Proliferation and Differentiation

4. Antigen Elimination and Memory

Stage 1: Antigen Presentation

• Antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells process and present antigens to T cells.

• APCs use Major Histocompatibility Complex (MHC) molecules to display antigen fragments:

  • MHC I: Present on all nucleated cells; presents intracellular antigens to CD8+ cytotoxic T cells.

  • MHC II: Present only on APCs; presents extracellular antigens to CD4+ helper T cells.

Type	MHC I	MHC II
Location	All nucleated cells (except RBCs)	APCs only
Interacts with	CD8+ T cells	CD4+ T cells
Antigens Presented	Intracellular	Extracellular

Stage 2: Lymphocyte Activation

• T cells require two signals for activation:

  • Primary: TCR binds to MHC-antigen complex.

  • Secondary: Co-stimulatory proteins (e.g., B7 on APC binds CD28 on T cell).

• B cells can be activated by direct antigen binding (T-independent) or require T helper cell assistance (T-dependent).

Stage 3: Lymphocyte Proliferation and Differentiation

• Activated lymphocytes undergo clonal expansion (proliferation) and differentiate into effector and memory cells.

• Effector cells: Actively participate in eliminating the antigen.

• Memory cells: Remain in lymphatic tissues for rapid response upon re-exposure.

Stage 4: Antigen Elimination and Memory

• Effector cells eliminate the antigen through cellular and humoral mechanisms.

• After the threat is cleared, most effector cells die, but memory cells persist for years, providing immunological memory.

T Helper Cell Subsets and Functions

• TH1 cells: Promote cellular responses by activating cytotoxic T cells, macrophages, and NK cells.

• TH2 cells: Stimulate B cells to produce antibodies (humoral response).

• T regulatory (Treg) cells: Suppress immune responses to maintain tolerance and prevent autoimmunity.

Cytotoxic T Cells (CD8+)

• Directly destroy infected, damaged, foreign, or cancerous cells.

• Mechanism: Release perforins (form pores) and granzymes (induce apoptosis) upon recognition of MHC I-antigen complex.

Humoral Immunity: B Cells and Antibodies

B Cell Activation

• T-independent activation: Certain antigens (e.g., polysaccharides) can activate B cells directly.

• T-dependent activation: Protein antigens require help from T helper cells (especially TH2) for full B cell activation.

• Activation involves antigen binding to BCR and co-stimulatory interactions (e.g., CD40 on B cell with CD40L on T cell).

B Cell Proliferation and Differentiation

• Activated B cells proliferate and differentiate into:

  • Plasma cells: Secrete antibodies specific to the antigen.

  • Memory B cells: Provide long-term immunity.

Antibody Functions

• Antibodies (immunoglobulins) help eliminate antigens by:

  • Neutralizing toxins and pathogens

  • Activating complement cascade (leading to cytolysis, inflammation, opsonization)

  • Enhancing phagocytosis (precipitation, agglutination, opsonization)

Antibody Structure and Isotypes

• Basic structure: Two heavy chains, two light chains, antigen-binding sites at the tips.

• Fc region determines the antibody's class and function.

• Isotype switching allows B cells to produce different classes of antibodies with the same antigen specificity.

Isotype	Structure	Proportion	Neutralization	Complement Activation	Opsonization	Agglutination/Precipitation	Half-life	Notes
IgG	Monomer	Most abundant	Strong	Strong	Strong	Strong	21 days	Crosses placenta
IgA	Monomer or dimer	Second most abundant	Strong	Some	Some	Negligible	6 days	Main antibody in mucous
IgM	Monomer or pentamer	Third most abundant	Strong	Strong	Negligible	Strong	10 days	First made in infection
IgE	Monomer	Rare	Negligible	Negligible	Negligible	Negligible	2 days	Allergy/parasite responses
IgD	Monomer	Rare	Negligible	Negligible	Negligible	Negligible	2 days	B cell receptor; poorly understood

Immunological Memory

• Memory cells (both T and B) persist in lymphoid tissues after the initial response.

• Upon re-exposure to the same antigen, memory cells mount a faster and stronger response (secondary response).

Example: The secondary antibody response is more rapid and robust than the primary response, providing the basis for long-lasting immunity after infection or vaccination.

*Additional info: The notes above integrate and expand upon the provided lecture slides, including definitions, mechanisms, and clinical relevance for adaptive immunity, T and B cell biology, and antibody function, as expected in a college-level microbiology course.*