Monocytes, Macrophages, B cells, Non-professional APCs
Why do we have many different types of APCs? Let me describe a few of them for you and maybe it will become clear.
The professionals of professional APCs are the DCs. They efficiently sample the environment for soluble as well as bigger antigens. Immature DCs are present in most tissues and once mature move rapidly to the nearest lymphoid tissue to present to awaiting T cells. They are the most effective APCs for presentation to T cells which have never encountered antigen before, the naive T cells. But what happens if we need more APCs during an inflammatory event? Have you noticed the monocytes in the blood vessel in the graphic on the right? Monocytes circulating in the blood travel rapidly to the inflamed tissue, where they differentiate into new DCs and macrophages that, in addition to having important innate functions, also are effective APCs that present antigens from phagocytosed pathogens to T cells in the tissue seen here. Blood monocytes are able to sample antigens in the blood before entering tissues where they then present these antigens.
What about a situation in which there is very little antigen about? For this we have B cells that take up soluble antigen through receptor-mediated endocytosis via its surface immunoglobulin (Ig) which is part of the B cell receptor (BCR). As you can see in the graphic, once the antigen, in this case a birch pollen, is bound to the Ig, the whole recector with antigen is internalised. The antigen is then directed into the endosomal pathway for antigen presentation. Can you see the similarity to allergen focusing via FcεRI? Other cell types which can present antigen under some circumstances include endothelial cells, epithelial cells and keratinocytes; which, when acting as APCs, have tissue specific specialized roles.
The diversity of APCs relates to the diversity of their locations and abilities. Some are skilled in taking up antigen in situations where the antigen concentration is low, others are better at dealing with particulate rather than soluble antigen. Some have tissue specific roles, while others are good at starting immune response by presenting to naive T cells. By the way, which APC was that? If you can't remember check-out the next page.