Monocytes are cells that circulate in the blood and become macrophages in the tissues. These cells are very important in alerting the immune system about an infection. 

Macrophages are scavengers whose job is to engulf or eat up infecting germs and even infected cells. Macrophages also help to overcome infection by secreting signals that help activate other cell types to fight against infections.

AAAAI - American Academy of Allergy Asthma & Immunology