What do Langerhan's cells, interdigitating cells, follicular dendritic cells, B cells and macrophages all have in common? Yep, they are all antigen presenting cells (APCs). APCs are a heterogeneous population of cells sourced from cells of both the innate and adaptive immune systems. Most APCs are probably derived from the phagocytic line but we are not really sure. Clearly B cells are a very different cell type, not phagocytic at all, and there may be other different cell populations. We can't make up our mind which cells are just different looking phagocytes and which are unique cell types. For example, when skin Langerhan's cells migrate to lymph nodes they become interdigitating cells. It's also possible to artificially make tissue cells in to APCs if we give them certain cytokine stimuli. We think that hair follicle cells in alopecia areata may start to present their own antigens to T cells in prolonged episodes of hair loss.
T cells and B cells are blind and need antigens to be presented to them. This is done by APCs expressing antigens on their cell surface in conjunction with a molecule called the Major Histocompatibility Complex (MHC something else we will have to talk about in more detail later) T cells then "see" the antigen using their TCRs and B cells see antigens via their cell surface antibody. APCs are found primarily in the skin lymph nodes spleen and thymus, the areas of high T or B cell concentration.
It is intriguing to note that B cells can both present antigen and be stimulated by it. Here we have cascade mechanism. Initially B cells are stimulated by antigen presented by other APCs. Once stimulated, the B cells themselves can join in antigen presentation to recruit even more B and T cells to the inflammatory reaction.
Well I think that covers all the immune system cells and their basic function. I hope you got all that! We have a wide range of cells with different functions. Cell functions for different clones overlap with other cell clone functions so that even if a part of the immune system is knocked out the immune system will still try to respond. For example if the entire adaptive immune system is knocked out, neutrophils will attempt to fill the gap as best they can. Their response is far from perfect but it does help. Knock out Tc cells and Th cells will proliferate to fill the gap. Knock out Th cells and both B and Tc cells will try to fill the gap and so on. Most of the cells in the adaptive immune system collect themselves into organs and that will be the subject of the next chapter. <!-- #EndEditable -->
