Final answer:
Macrophages are the type of cells that express both MHC class I and II molecules and function as professional antigen-presenting cells capable of activating T cells by presenting antigens.
Step-by-step explanation:
The type of cells that express both MHC class I and MHC class II molecules and can function as professional antigen-presenting cells (APCs) are macrophages. MHC class II molecules are expressed exclusively on the surface of certain immune cells that are specialized in presenting antigens to T cells to trigger an immune response. The three main types of professional APCs that express MHC class II molecules are macrophages, dendritic cells, and B cells. Thus, macrophages are the correct answer to the student question, as they are capable of both phagocytizing pathogens and presenting antigens using both MHC class I and MHC class II to initiate and modulate the immune response.
Macrophages are known for their role in stimulating T cells via the release of cytokines and enhancing phagocytosis, an essential part of the immune system's defense mechanism. Every nucleated cell in the body presents MHC class I, which aids in the presentation of normal self-antigens, as well as nonself pathogen antigens to effector T cells. In contrast, MHC class II molecules, which are crucial for the activation of helper T cells, are found only on macrophages, dendritic cells, and B cells, hence defining their role as professional APCs.