They are structurally and evolutionarily related to immunoglobulins. T cells recognize antigen only in the context of other proteins found on cell surfaces. Section 20.7 Proteins encoded by the MHC are molecular reference markers that permit T cells to interact with antigen. Found on the surfaces of APCs and target cells, MHC proteins embed processed antigen, and present it to T cells. This is the only way T cells can recognize nonself antigens, and initiate the specific immune response.
Cytokines are soluble mediators, produced by leukocytes that regulate interactions between cells.
Others, such as IFN and TNF, act on a wide variety of cell types. Chemokines are also soluble mediators, but are produced by a variety of cell types in response to injury. Activated T cells may interact directly to kill target cells, or they may facilitate involvement of other cellular effector cells, particularly macrophages, with cytokines. The sole function of the thymus is to produce T cell clones that react only with nonself antigens. Most self reactive T cells are deleted during development and maturation of the thymus.