T cells, on the other hand, do not secrete their receptors, called simply T cell receptors (or TCR for short). Although the TCR also can bind to antigens, these antigens must be proteins. Further, the TCR cannot detect native proteins, but they must be degraded to short (8-20 amino acids long) peptides and presented bound to cell-surface molecules called the MHC (short for Major Histocompatibility Complex) molecules. The MHC molecules are what doctors use to determine tissue types for transplants and grafts.
There are two different types of T cells, identified by the cell-surface proteins CD4 and CD8, and they serve different functions. CD8+ T cells, also called CTL's (cytotoxic T lymphocytes) or killer T cells, are mainly involved in killing virus-infected and tumor cells. CD4+ T cells, called helper T cells, are responsible for regulating the immune response. They secrete growth factors needed for B cell (antibody) activity and for optimal CTL activity. They are also involved in fine tuning the immune response between antibody production and CTL activation, since different pathogens have different susceptibilities to these two types of attack. There is also increasing evidence that CD4+ T cells play a role in turning off an immune response when the danger is gone.
Since T cells only see antigens in the context of MHC molecules, and there are two different types of T cells, it should not be surprising that there are also two types of MHC molecules: Class I and Class II. Class I MHC molecules, which are expressed on essentially all nucleated cells, present peptides derived almost exclusively from intracellular proteins, and these are recognized by CD8+ T cells. Class II MHC molecules are expressed only on a limited subset of cells (including macrophages, B cells, dendritic cells, and some glial cells) and present peptides to CD4+ T cells. The Class II pathway is responsible for processing and presenting extracellular antigens, although it is clear that intracellular proteins can also enter the pathway.