T-cell activation

The immune response is highly specific for each invader, and that’s because the cells of the adaptive immune response have receptors that can differentiate friendly bacteria from potentially deadly pathogens from their unique parts - called antigens.

The key cells of the adaptive immune response are the lymphocytes - the B and T cells.

T cells develop in the thymus where they undergo a process called VDJ rearrangement to generate a massively diverse set of T cell receptors, or TCRs.

There are two types of T cells which are identified based on molecules they express on their surface.

Helper T cells express CD4 on their surface, and their main function is to support other immune cells.

Cytotoxic T cells express CD8 on their surface, and their main function is to kill infected or cancerous cells.

A T cell starts out naive and then gets primed or activated - at which point it differentiates into an effector T cell and proliferates. Activation of both helper and cytotoxic T cells requires two signals.

After that the cytokines present around the cell determine the type of T cell it will become. The first signal occurs when a T cell receptor binds to an antigen.

Now, a T cell receptor can only recognize antigens that are peptides, rather than carbohydrates or lipids.

And the T cell receptor also needs to have a peptide presented on a major histocompatibility complex, also known as MHC.

MHC molecules act like serving platters present the antigen to T cells.There are two types of MHC molecules that work with the two types of T cells.

MHC class I molecules present antigen to CD8+ T cells and MHC class II molecules present antigen to CD4+ T cells.

MHC class II molecules are found on the surface of an antigen presenting cell like a macrophage or dendritic cells while MHC class I molecules are found on all nucleated cells throughout the body.