Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
Human Leukocyte Antigen, or HLA, is a class of protein which is often found on the surface membrane of cells, and which serves the purpose of "presenting" possible antigen to T and B cells. There are different sorts of HLA - class I HLA and class II HLA - and they each work slightly differently.
Both do essentially the same job though. They sit inside a cell, and have a groove in which they can attach little bits of protein. (These bits of protein will have come from outside the cell or could be products of the cell's own genes.) Once a bit of protein (or other possible antigen) is attached, the HLA moves out and sits on the outside of the cell, so that a T cell can come along and check the molecule that is being presented. The T cells, in their turn, check whether the molecule is "self" or foreign, and will either pass over it or react to its presence (either destroying the cell, or raising the alarm about the foreign substance by emitting cytokines">cytokines).
Class I HLA presents antigen peptide found within the cell, to CD8%20cell%20surface%20protein">CD8 cell surface protein (i.e. normally to cytotoxic T cells).