Antigen presentation. Computer illustration of an antigen-presenting cell (APC) displaying an antigen fragment to a T cell (T lymphocyte). APCs, or accessory cells, are an important part of the immune response. They process antigens, displaying them on their cell surface, complexed with major histocompatibility complexes (MHCs), and present them to T cells (T lymphocytes). This process, known as antigen presentation, is vital for T cells to recognise the antigen and deal with it effectively.