Final answer:
Antigen-presenting cells (APCs), which include macrophages, dendritic cells, and B cells, are crucial for the immune system, phagocytizing pathogens and presenting antigens with MHC class II molecules to T cells to activate them.
Step-by-step explanation:
The class of cells that engulf pathogens and then can present antigens on their surface to activate the specific, cell-mediated response are known as antigen-presenting cells (APCs). These include macrophages, dendritic cells, and B cells. They are very effective phagocytic cells that not only engulf pathogens and worn-out cells but also serve a critical role in the immune system by displaying processed antigens on their surface in conjunction with Major Histocompatibility Complex (MHC) class II molecules. This process is essential for the activation of T cells, which are a cornerstone of the adaptive immune response. When a pathogen is phagocytized and digested, a part of it, usually a protein fragment known as an epitope, is presented on the APC's surface, where it can be recognized by T cells, leading to their activation.
Professional APCs display ingested antigens with MHC II molecules to T cells. This exposure helps T cells, particularly helper T cells, become fully activated, a necessary step in coordinating the body's response to invaders. Both macrophages and dendritic cells are proficient at engulfing pathogens through phagocytosis and then proceeding to stimulate T cells by presenting the pathogen's antigens. This involvement of professional APCs constitutes an important bridge between the innate and adaptive arms of the immune system.