The key cells of the adaptive immune response are the lymphocytes - the B and T cells.
And there are two types of T cells.
Helper T cells which express CD4 on their surface, and cytotoxic T cells which express CD8 on their surface.
Helper T cells primarily support other immune cells, whereas cytotoxic T cells kill cells that are infected with a pathogen or are cancerous.
Cytotoxic T cells, along with natural killer cells, are part of cell mediated immunity.
Cell mediated immunity refers to the part of the immune response that’s based on cellular interactions, and cannot be transferred through serum from one person to another.
That makes sense since both natural killer and cytotoxic T cells need to interact directly with a target cell in order to destroy it.
Now, when a T cell is initially formed it’s considered naive.
Later when that T cell encounters an antigen in the lymph node- it gets activated or primed - and turns into an effector T cell.
This process of priming requires two signals.
The first signal is the antigen itself, which is usually presented on an MHC molecule on the surface of an antigen presenting cell like a macrophage or dendritic cell.
Cytotoxic T cells respond to intracellular antigens - like viruses, intracellular bacteria, and tumor antigens.
The naive cytotoxic T cell needs a high level of stimulation to become activated and it relies on a process called cross-presentation to reach that level.
In cross-presentation macrophages or dendritic cells take up the antigen and then present it to the cytotoxic T cell.
These antigens typically come from extracellular pathogens or from tumor cells or virally infected cells.