A) permit certain innate immune system cells to recognize certain molecules that are unique to microorganisms. B) are found on the surface of cytotoxic T cells, and are used to recognize infected cells. C) present foreign antigens on the surface of macrophages and dendritic cells. D) are found on the surface of helper T cells, where they recognize antigens presented by antigen presenting cells. E) are used by macrophages and dendritic cells to signal helper T cells that an infection is under way.