Final answer:
Antigen-presenting cells (APCs) such as macrophages, dendritic cells, and B cells present antigens to T cells using MHC class II molecules, thereby activating the T cells for the adaptive immune response.
Step-by-step explanation:
Antigen-presenting cells (APCs) present fragments of cells so that they can be recognized by T cells. APCs such as macrophages, dendritic cells, and B cells engulf pathogens and break them down. After this, the antigen fragments are loaded onto MHC class II molecules and brought to the surface of the APC. There, they are presented to T cells, which are crucial for the adaptive immune response. The T cells are then activated and can carry out their role in attacking the pathogen.