These are receptors found on the surface of cells that react with the substance Chemokine. In humans we have 19 types of these receptors. The interaction with the specified protein causes a reaction in the cell called chemotaxis
Chemokine Receptor 5 is the protein that is located on the surface of white blood cells and therefore a part of the immune system. The HIV virus uses this site on the cell wall to enter the cell and invade it.
Humans have been shown to have a mutation in their genetic code that deletes a portion of this sequence to createthe CCR5 receptor and therefore HIV is not able to enter the cell - resistance to HIV. Scientists think that this evolved in the last 2,000 years in response to an infectious agent that used that particular site to invade the cells.
Murphy PM, Baggiolini M, Charo IF, Hébert CA, Horuk R, Matsushima K, Miller LH, Oppenheim JJ, Power CA (2000). "International union of pharmacology. XXII. Nomenclature for chemokine receptors" (abstract page). Pharmacol. Rev. 52 (1): 145–76
Stock, J. 2008. Are Humans Still Evolving? European Molecular Biology Organization. 9,551-554.