A unique feature of T cells is their ability to discriminate between peptides derived from healthy, endogenous cells and peptides from foreign or abnormal (e.g. infected or cancerous) cells in the body. Antigen presenting cells do not discriminate between self and foreign peptides and typically express a large number of self derived pMHC on their cell surface and only a few copies of any foreign pMHC. For example, it has been shown that cells infected with HIV have only 8-46 HIV specific pMHCs next to 100000 total pMHC per cell.