Final answer:
CR2 is expressed on B cells, which are important in the immune system for antigen presentation and activation. MHC I molecules are recognized by CTLs with CD8 coreceptors, while MHC II molecules are found on macrophages, dendritic cells, and B cells. Platelets, along with other cells, are derived from myeloid stem cells, and macrophages serve as the main phagocytic cells in the immune system.
Step-by-step explanation:
CR2, also known as complement receptor type 2, is expressed primarily on B cells. The CR2 receptor plays a significant role in the immune system, including B cell activation and regulation. In the context of antigen-presenting molecules and immune cell functions, MHC I molecules are found on all nucleated cells, and are the type of antigen-presenting molecule that are recognized by CD8 coreceptors on CTLs (cytotoxic T lymphocytes), which are responsible for cellular adaptive immunity. In contrast, MHC II molecules are found only on macrophages, dendritic cells, and B cells, and are essential for the initiation of immune responses by presenting antigens to helper T cells.
As for other hematopoietic cells, platelets, along with other formed elements such as erythrocytes and leukocytes, arise from myeloid stem cells. The phagocytic capabilities in the immune system are typically associated with cells such as macrophages, which are adept at engulfing and digesting pathogens.