It will be explained below that these three types of APC's present different sets of antigens and also may serve to activate T cells at different points during the immune response. The most relevant property of APC’s is that, in addition to antigen presentation, they provide co-stimulatory signals via B7.1 and B7.2. It is important to note that APC's do not constitutively express these co-stimulatory molecules. (Except Dendritic cells, see below). Since these cells potentially phagocytose both self and infectious materials, there has to be some mechanism for the recognition of infection (non-self). Upon this recognition, the APC's will up regulate their co-stimulatory molecules (namely B7's), and only then will they activate T cells, by interactiong with the B7 receptor called CD28.

1)    MACROPHAGES

• These cells are part of the innate response. Unlike T and B cells, they do not contain any specific receptors. Macrophages continuously phagocytose self-proteins and cells in their vicinity, during normal tissue repair and aging (e.g. old red blood cells). All of these proteins are degraded and presented on MHC-II. These self-proteins however, do not activate T cells, because in the absence of infection, macrophages express low levels of MHC-II, and almost no co-stimulator (B7). Further, T cells with high affinity receptor for self-peptides have been deleted during T cell development in the thymus.
  In the case of infection, however, macrophages posses certain types of receptors that recognize differential carbohydrate patterns on foreign cells. They also have receptors for specific bacterial products such as lipopolysaccharide (LPS) (endotoxin). When these molecules bind their bacterial ligands, they stimulate the macrophages to up regulate MHC-II and B7, providing these cells with strong antigen presentation properties. They also start to secrete Cytokines   that aid in their functions. (note: IL-1, 6, 8, 12 and TNF-a). It is at this point that antigen presentation by MHC II will activate Th cells.

• See PPD (tuberculin) test for Mycobacterium Tuberculosis exposure.

• Since viral proteins are internally synthesized by host cells, they possess the same carbohydrate pattern as the host. Thus macrophages do ingest them as part of the general tissue debris, present the antigen as peptides, but do not become activated.

• These cells are mostly found in the skin and mucosal epithelium, where they are referred to as Langerhan's cells. These cells continuously express high levels of co-stimulatory B7. During ingestion of self proteins, they remain in the epithelium, inaccessible to "naïve" T cells. Naive T cells circulate through blood and the lymphatic system. Only activated and/or memory T cells can migrate to tissues.
  Upon recognition of infectious particles, these cells migrate through the lymphatics to the nearest lymph node (Dendritic cells also possess non-specific receptors that can recognize infectious particles). In the follicles of the lymph node they come into close contact with naive T cells.  Unlike macrophages however, Dendritic cells can also recognize viral particles as non-self.  In addition, they can present antigen via both MHC I and MHC II. Thus they can activate both CD8 and CD4 T cells, directly. Once the T cells are activated, they will leave the lymph nodes and travel to the sites of inflammation. Note that since Dendiritic cells present viral particles, they should also activate CD8 cells, the main effector cell to fight viral infections (intracellular). Dendritic cells are also very numerous in the thymus, where they act in positive and negative selection during T cell development.