Major histocompatibility complex.... Actually it is a very interesting to know that on our all cells of body MHC protein is present except RBC and these are of two types.... On APC both MHC protein are present and there these are used to save memory of foreign microorganisms also.... When macrophages digest the microorganism it show a MHC protein on its surface with which memory cells strike and take the protein impression on it..... This is all that i know....