MHC II / Helper T Cell Interactions.
The helper T cell (TH cell) is responsible for regulating almost every acquired immune system activity. The signaling molecules on its membrane and the cytokines it producesare necessary for the development of most acquired immune responses. Just as CTLs must interact with MHC I, helper T cells must contact a MHC II-peptide complex which they recognize in order to be activated. Instead of CD8, helper T cells generally have a molecule called CD4 which binds to MHC II. As in the case of CTLs, self-reactive cells are destroyed in the thymus or tolerized in the periphery, so if a peptide is recognized, it is probably of foreign origin. Assuming the APC which presented the MHC II-peptide also possessed a surface molecule or cytokine which acted to co-stimulate the TH cell, the cell will produce cytokines to allow its own maturation and propagation.

These helper T cells will then proceed to supply the necessary cytokines for immune responses against the antigen which they recognize. Both humoral and cytotoxic immunity are seriously hindered without this support. In some cases, as in the immune response to tuberculosis, the helper T cells activation of the innate immune system is the primary immune response. As with CTLs, a portion of helper T cells are memory cells which remain dormant until a reinfection occurs.
Conclusions

The importance of the T cell in the immune system, particularly with regards to intracellular infections and tumors is clear. The MHC molecules appear to be the keys to understanding and manipulating both helper and cytotoxic T cells. Though our understanding of the TCR is limited, we have a great deal of knowledge concerning the presentation of peptides by MHC molecules. A reliable method for predicting the peptides which can bind to MHC, and which therefore may be able to induce T cell immunity, would be of great value. In the coming years, improved techniques for making such predictions will improve our understanding of T cells and immunity in general. With that improvement will come an ability to develop superior means of modulating the immune response.