What type of cells circulate in the blood as macrophages?

Monocytes are a type of white blood cell that circulate in the bloodstream and play a crucial role in the immune response. When monocytes migrate from the blood into tissues, they differentiate into macrophages. This conversion allows them to effectively engulf and digest pathogens, dead cells, and cellular debris, which is integral to the body's defense mechanisms. Macrophages are essential for both innate and adaptive immunity, as they help to initiate inflammation and provide signals that activate other immune cells.

Neutrophils, while also important in the immune response, are more specialized for attacking and destroying bacteria through phagocytosis and do not differentiate into macrophages. B cells and T cells are types of lymphocytes involved in adaptive immunity, with B cells producing antibodies and T cells playing various roles, such as killing infected cells or aiding other immune cells. Thus, the direct lineage from monocytes to macrophages highlights why this choice is the correct answer.