Before the immune system can respond to an antigen it must be able to recognize the antigen. It is able to do so through antigen processing. Macrophages are the major antigen processing cells others include B-lymphocytes.

The breakdown of antigenic proteins into smaller fragments, which are then presented on the cell surface, along with MHC proteins, to T cells.

the degradation of proteins into peptides that can bind to MHC molecules for presentation to lymphocytes.