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...

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:

1. Phagocytose and kill pathogens in the blood


2. Line the endothelial surfaces of the blood vessels with phagocytic cells


3. Enter lymph nodes and patrol for infecting microbes in these organs


4. Amplify the local innate immune response by entering tissues that are infected


5. Differentiate into dendritic cells during an inflammatory response