There are 3 main types of T cells: cytotoxic, helper, and regulatory. Each of them has a different role in the immune response.

Cytotoxic T cells (CD8+)

Cytotoxic T cells (Tc cells) have a co-receptor called CD8 on their cell surface. CD8 partners with the T cell receptor and with MHC class I molecules, acting as a sort of bridge. This bridge allows cytotoxic T cells to recognize normal cells that are infected by a pathogen. When the cytotoxic T cell recognizes the infected cell, it becomes activated and produces molecules that kill the infected cell, destroying the pathogen in the process.

Helper T cells (CD4+)

Helper T cells (Th cells) have a different co-receptor called CD4 on their cell surface. CD4 also partners with the T cell receptor but interacts with MHC class II molecules instead of MHC class I molecules. This allows helper T cells to recognize pathogen peptides that have been displayed by antigen presenting cells. When helper T cells recognize a peptide on an antigen presenting cell, they become activated and begin to produce molecules called cytokines that signal to other immune cells.

There are many subtypes of helper T cells (ie, Th1, Th2, Th17). Each subtype produces a specialized combination of cytokines that depends on type of pathogen that the helper T cell has recognized—some cytokines are more effective than others in the process of eliminating certain invaders.

Regulatory T cells

Regulatory T cells (Treg cells) also have CD4 on their surface, but they do not activate the immune system like helper T cells do. Instead, regulatory T cells play a protective role by shutting off the immune response when it is no longer needed. This prevents excessive damage to the normal cells and tissues in the body. Regulatory T cells suppress the immune response in several ways, including:

• Producing anti-inflammatory cytokines that suppress the immune response

• Releasing molecules that kill activated immune cells

• Changing the way dendritic cells behave so they can't activate T cell