What type of blood cell is a macrophage?

A macrophage is a type of white blood cell that originates from monocytes, which are a particular category of leukocytes. When monocytes leave the bloodstream and enter tissues, they differentiate into macrophages. These cells play a critical role in the immune response by engulfing and digesting pathogens, dead cells, and cellular debris. They are essential for both innate and adaptive immunity, acting as phagocytes that help to clean up sites of infection or injury. This connection between monocytes and macrophages highlights the critical role of monocytes in the immune response, making the correct answer C.

Understanding this relationship clarifies why the other choices do not correctly answer the question: platelets are involved in clotting, lymphocytes are primarily responsible for adaptive immune responses, and erythrocytes are red blood cells involved in oxygen transport. None of these options have the functionality that defines macrophages.