Antigen-presenting cells (APCs) are cells that participate in antigen-presentation to T cells. consequently their function as APCs is that they partially digest an antigen, complex it with major histocompatibility complex (MHC) molecules, of either variety (MHC I or MHC II) which can then be identified by the T cell receptor complex in the T cell's surface. There are three major types of APCs known (which are listed below in descending order of antigen-presenting ability):
Dendritic cells are the most powerful APCs, and antigen-presentation is actually their chief purpose in life. Macrophages are primarily phagocytic cells although they can serve as APCs too. B cells usually only serve as APCs to helper T cells in order to enable their own activation into plasma cells.