Introduction
While learning about antigen processing and presentation in the previous pages, you might have started wondering what an antigen presenting cell really is. After all, we have just told you that all our cells are able to present antigen to our cytotoxic T cells via MHC class I. Are all our cells therefore antigen presenting cells? Well yes, as they do present antigen in this way, but at the same time no! When we talk about antigen presenting cells we mean specialised cells able to present antigens from their surroundings. In other words cells that express MHC class II.
Many types of cells are antigen presenting cells. Broadly they can be classed as professional APCs, if they always express MHC class II, or non-professional APCs, if they are induced to express MHC class II. Examples of professional APCs include DCs, macrophages and B cells, while non-professional cells include epithelial cells, endothelial cells and glial cells.
So why do we need this diversity of APCs? Find out on the next few pages.