Most normal tissues contain resident macrophages, and connective tissue sites in the gastrointestinal tract and the lung contain large numbers of these cells. Yet the blood also contains a high number of circulating 'classical' monocytes that can differentiate into macrophages after entering tissues. These circulating monocytes function to:
A) Phagocytose and kill pathogens in the blood
B) Line the endothelial surfaces of the blood vessels with phagocytic cells
C) Enter lymph nodes and patrol for infecting microbes in these organs
D) Amplify the local innate immune response by entering tissues that are infected
E) Differentiate into dendritic cells during an inflammatory response

Question

Quizplus · Accepted Answer

Circulating monocytes can differentiate into macrophages after entering tissues and amplify the local innate immune response in the infected tissues. This helps in clearing the infection and promoting tissue repair. Option A is incorrect as monocytes don't phagocytose and kill pathogens in the blood, but they help in clearing pathogens in the infected tissues. Option B is incorrect as endothelial cells themselves have intrinsic phagocytic properties, and monocytes do not line endothelial surfaces. Option C is incorrect as lymph nodes have resident immune cells and do not require patrols from circulating monocytes. Option E is incorrect as dendritic cells are differentiated from monocytes but not during an inflammatory response.

Most Normal Tissues Contain Resident Macrophages, and Connective Tissue Sites