Cell-mediated immunity

There are two main classes of T-lymphocytes:

• Cytotoxic T cells
• Helper T cells

Cytotoxic T cells recognize cells that are infected with a virus or other intracellular microorganism. Such foreign proteins, which invade a cell as an infection, become partially intracellular dismantled and finally - with the help of MHC-proteins transported to the cell surface where they are presented to the cytotoxic T cells. These kill the host cell in a direct way in that they trigger a programmed cell death.
The helper T cells, on the other hand, stimulate the immune reaction of other cells; for example, they help in the activation of macrophages and B cells

Both types of T cells express antibody-like receptors on the plasma membrane. These are coded by genes that are put together out of many gene segments during the maturation phase of the T cells in the thymus (see also More info: Interactive diagram). Such receptors recognize antigen fragments that are presented on the host cell's surface in connection with the MHC molecules (major histocompatibility-complex). (see: Development of immunologic tolerance).