Well-Trained Immune Cells Keep HIV in Check

The human immune system detects foreign cells with the help of cell-surface proteins called human leukocyte antigens (HLAs). Each person's cells carry a particular set of HLA molecules — the person's HLA type — which bind fragments of virus or bacterial protein and 'present' them to T cells, the immune cells that recognize and attack infected cells. But before T cells are ready to perform their killer function, they are in effect trained on fragments of the body's own proteins — self-peptides — in an organ called the thymus. To 'graduate' from the thymus, a T cell must be able to recognize at least one combination of HLA molecule and self-peptide, which provides the template for its subsequent immune response against a foreign peptide bound to that HLA molecule. T cells that bind to self-peptides very strongly, however, are rejected, as they would attack the body's own cells.

http://www.nature.com/news/2010/100505/full/news.2010.219.html