T Cell Activation and Effector Functions

Clonal Expansion

Once lymphocytes are activated, they replicate multiple times a day for 3 to 5 days. Therefore, one naïve lymphocyte can give rise to about 1000 daughter cells of the same antigen specificity.
During this process of clonal expansion, they receive cytokine signals and differentiate into effector T cells. They upregulate chemokines and adhesion molecules that guide them to sites of infection.
Effector cells have a limited life span. One antigen is removed, most of the antigen-specific cells undergo apoptosis.
However, some of the lymphocytes persist after antigen is removed. These cells form memory T cells, which ensure a more rapid and effective response when the individual encounters a pathogen in the future.

Antigens derived from replicating viruses are displayed on the surfaces of infected cells in the context of MHC Class I molecules, and are recognized by cytotoxic CD8+ cells.
Cytotoxic T cells recognize the complex of viral peptide antigen and MHC class I and kill the infected cell that expresses the antigen.

The T cells that express CD4 play various roles in the immune response. They differentiate into 4 subsets (Th1, Th2, Th17 and T regulatory cells) based on the cytokine signals that they receive, and each of those subsets is specifically tailored to fight a different form of pathogen.

Th1 cells play important roles in the control of intracellular bacterial and viral infections.
One of their most important functions is that they secrete IFN-gamma to activate macrophages.
This causes macrophages to fuse their lysosomes with the vesicles containing bacteria, which leads to the destruction of the intracellular bacteria.

Th2 cells play a key role in the immune response to helminth infections. They express CD40L which binds to CD40 on the B cell, leading to the directed release of cytokines such as IL-4 and IL-13. Toegether, these functions allow Th2 cells to stimulate B cells to proliferate and differentiate into antibody-secreting cells.

Th17 cells are important in mucosal bacterial and fungal responses.
They are characterized by the production of the cytokines interleukin-17 (IL-17) and interleukin-22 (IL-22).

Treg cells are CD4+ T cells that are immunosuppressive.
They are characterized by their expression of forkhead transcription factor FOXP3 and produce inhibitory cytokines such as transforming growth factor beta (TGF beta) and IL-10.