Professional APCs are very efficient at internalizing antigen, either by [[phagocytosis]] or by receptor-mediated [[endocytosis]], and then displaying a fragment of the antigen, bound to a class II [[Major histocompatibility complex|MHC]] molecule, on their membrane. The T cell recognizes and interacts with the antigen-class II MHC molecule complex on the membrane of the antigen-presenting cell. An additional co-stimulatory signal is then produced by the antigen-presenting cell, leading to activation of the T cell. The expression of co-stimulatory molecules is a defining feature of professional APCs.
There are three main types of professional antigen-presenting cell:
* [[Dendritic cell]]s (DCs), which have the broadest range of antigen presentation, and are probably the most important APC. Activated DCs are especially potent T<sub>H</sub> cell activators because, as part of their composition, they express [[co-stimulation|co-stimulatory]] molecules such as [[B7 (protein)|B7]].
* [[Macrophage]]s, which are also CD4+ and are therefore also susceptible to infection by [[HIV]].
Certain [[B -cells]] , which express (as B cell receptor) and secrete a specific antibody, can internalize the antigen, which bind to its BCR and present it incorporated to MHC II molecule, but are inefficient APC for most other antigens.
* Certain activated [[epithelial cells]]