Cell-Mediated Immunity: T Cell Activation and Effector Functions

Cell-Mediated Immunity

Overview of T Cell Development and Activation

Cell-mediated immunity is a critical component of the adaptive immune response, primarily involving T lymphocytes. This process includes the development, activation, and effector functions of T cells, which are essential for defending the body against intracellular pathogens and regulating immune responses.

• T cell maturation occurs in the thymus, where immature T cells undergo selection to ensure self-tolerance and functional competency.

• Positive and negative selection remove T cells that do not recognize self-MHC or that react strongly with self-antigens.

• Mature T cells migrate to peripheral lymphoid organs, such as lymph nodes, where they can be activated by antigen-presenting cells (APCs).

• Activated T cells proliferate and migrate to sites of infection to eliminate pathogens.

Key Terms:

• Thymus: Primary lymphoid organ where T cell development and selection occur.

• Peripheral lymphoid organs: Secondary sites (e.g., lymph nodes, spleen) where mature T cells encounter antigens.

• Antigen-presenting cells (APCs): Cells such as dendritic cells, macrophages, and B cells that present antigens to T cells via MHC molecules.

Stages of T Cell Development and Activation

• Progenitor cell differentiation: A single progenitor cell gives rise to a diverse population of lymphocytes with unique antigen receptors.

• Removal of self-reactive lymphocytes: T cells that strongly recognize self-antigens are eliminated during negative selection.

• Naive T cell pool: Remaining T cells are released into circulation, ready to respond to foreign antigens.

• Clonal expansion: Upon activation by antigen, T cells proliferate and differentiate into effector cells.

Types of Effector T Cells

Classification and Functions of Effector T Cells

Effector T cells are specialized subsets that perform distinct roles in immune defense. Their differentiation is guided by the nature of the antigen and cytokine environment.

Effector T Cell Type	Main Function	Target Pathogens
CD8 Cytotoxic T Cells (CTL)	Kill virus-infected cells	Viruses (e.g., influenza, rabies), intracellular bacteria
CD4 TH1 Cells	Activate infected macrophages, help B cells for antibody production	Microbes that persist in macrophage vesicles (e.g., Mycobacterium tuberculosis)
CD4 TH2 Cells	Provide help to B cells for antibody production, especially switching to IgE	Helminth parasites
CD4 TH17 Cells	Enhance neutrophil response, promote barrier integrity	Klebsiella pneumoniae, fungi (e.g., Candida albicans)
TFH Cells	B cell help, isotype switching, antibody production	All types
Regulatory T Cells (Treg)	Suppress immune responses, maintain tolerance	Self-antigens, regulation of all types

Examples and Applications

• CD8 CTLs are crucial for eliminating cells infected with viruses or certain intracellular bacteria.

• CD4 TH1 cells help activate macrophages to destroy pathogens residing within vesicles.

• CD4 TH2 cells are important for defense against parasitic worms and for promoting IgE-mediated responses.

• CD4 TH17 cells enhance the recruitment of neutrophils and strengthen mucosal barriers against extracellular pathogens.

• TFH cells support B cell maturation and antibody production in germinal centers.

• Regulatory T cells prevent autoimmunity by suppressing excessive immune responses.

T Cell Activation in Secondary Lymphoid Tissues

Role of Dendritic Cells in T Cell Activation

T cells are activated in secondary lymphoid tissues, such as lymph nodes, primarily by dendritic cells. This process is essential for initiating adaptive immune responses.

• T cell circulation: Naive T cells continuously circulate between different patches of secondary lymphoid tissue, searching for their specific antigen.

• Dendritic cell function: Dendritic cells sense inflammation, phagocytose pathogens, and present processed antigens on MHC II (or MHC I if the pathogen is cytosolic).

• Migration: After capturing antigens, dendritic cells migrate to lymph nodes to find and activate matching T cells.

Example: Dendritic cells in the skin take up bacterial antigens and travel via lymphatic vessels to the nearest lymph node, where they present the antigen to T cells.

Key Steps in T Cell Activation

• Antigen recognition: T cell receptors (TCRs) bind to specific antigen-MHC complexes on APCs.

• Co-stimulation: Additional signals (e.g., CD28 on T cells binding B7 on APCs) are required for full activation.

• Cytokine signaling: APCs produce cytokines that influence T cell differentiation into various effector subsets.

Equation:

$\text{T cell activation} = \text{TCR recognition of antigen-MHC} + \text{Co-stimulatory signals} + \text{Cytokine environment}$

Summary Table: T Cell Activation Signals

Signal	Source	Function
Signal 1	Antigen-MHC complex (APC)	TCR recognition
Signal 2	Co-stimulatory molecules (e.g., CD28/B7)	Prevents anergy, ensures activation
Signal 3	Cytokines (e.g., IL-12, IL-4)	Directs T cell differentiation

Additional info:

• These notes are based on immunology concepts, not organic chemistry. The content is relevant for students studying cell-mediated immunity, T cell biology, and adaptive immune responses.