Antigen-presenting cells can be divided into two categories:
- professional antigen-presenting cells
- and
- non-professional antigen-presenting cells
Those that express MHC class II molecules together with co-stimulatory molecules and pattern recognition receptors (PPR) are often referred to as professional antigen-presenting cells. The non-professional APCs express MHC class I molecules. "Professional" in this context means that antigen presentation is an essential function of these cells.