Final answer:
Professional APCs, such as macrophages, dendritic cells, and B cells, express MHC class II molecules to present antigens and activate T cells, while most non-professional APCs do not.
Step-by-step explanation:
Both professional and non-professional Antigen-Presenting Cells (APCs) have distinct roles in immune responses. The question whether both professional and non-professional APCs express MHC class II can be addressed by understanding that MHC class II molecules are specifically expressed on the surface of professional APCs. These professional APCs include macrophages, dendritic cells, and B cells, which present abnormal or nonself pathogen antigens for the initial activation of T cells. In contrast, most non-professional APCs do not express MHC class II. As such, professional APCs can be differentiated from other cells by their ability to express class II MHC and subsequently activate helper T cells (TH) and regulatory T cells which are crucial for immune response.