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 recognizing and eliminating infected or abnormal cells.

• T cell maturation: T cells originate from progenitor cells and undergo selection processes in the thymus to ensure self-tolerance and functional competence.

• Activation: Mature T cells migrate to peripheral lymphoid organs, where they are activated by antigen-presenting cells (APCs).

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

Key Steps in T Cell Development:

• Positive and negative selection in the thymus removes self-reactive T cells.

• Mature T cells enter the pool of naive lymphocytes, ready to respond to foreign antigens.

• Upon activation, T cells undergo clonal expansion and differentiation into effector cells.

Equation for Clonal Expansion:

$\text{Number of effector T cells} = \text{Number of activated T cells} \times \text{Proliferation rate}$

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 cytokines and the nature of the antigen encountered.

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 survive in macrophages (e.g., Mycobacterium tuberculosis), extracellular bacteria
CD4 TH2 Cells	Provide help to B cells for antibody production, especially 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 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, by dendritic cells (DCs) that present antigens. This process is essential for initiating adaptive immune responses.

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

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

• Migration: DCs migrate to lymph nodes to find and activate matching T cells.

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

Summary of T Cell Priming and Effector Activity

Stages of T Cell Response

The T cell response can be summarized in three main stages: priming/activation, clonal expansion, and effector activity.

• Priming/activation: T cells encounter antigen-presenting cells and become activated.

• Clonal expansion: Activated T cells proliferate, increasing the number of effector cells.

• Effector activity: Effector T cells migrate to sites of infection and eliminate pathogens through cytotoxicity or by helping other immune cells.

Equation for T Cell Activation:

$\text{T cell activation} = \text{Antigen recognition} + \text{Co-stimulatory signals}$

Additional info: The notes and images provided are focused on immunology and cell-mediated immunity, which is not directly part of a standard Organic Chemistry curriculum. However, the content is highly relevant for students studying immunology, cell biology, or biochemistry.