<P> Phagocytes of humans and other jawed vertebrates are divided into "professional" and "non-professional" groups based on the efficiency with which they participate in phagocytosis . The professional phagocytes are the monocytes, macrophages, neutrophils, tissue dendritic cells and mast cells . One litre of human blood contains about six billion phagocytes . </P> <P> All phagocytes, and especially macrophages, exist in degrees of readiness . Macrophages are usually relatively dormant in the tissues and proliferate slowly . In this semi-resting state, they clear away dead host cells and other non-infectious debris and rarely take part in antigen presentation . But, during an infection, they receive chemical signals--usually interferon gamma--which increases their production of MHC II molecules and which prepares them for presenting antigens . In this state, macrophages are good antigen presenters and killers . However, if they receive a signal directly from an invader, they become "hyperactivated", stop proliferating, and concentrate on killing . Their size and rate of phagocytosis increases--some become large enough to engulf invading protozoa . </P> <P> In the blood, neutrophils are inactive but are swept along at high speed . When they receive signals from macrophages at the sites of inflammation, they slow down and leave the blood . In the tissues, they are activated by cytokines and arrive at the battle scene ready to kill . </P> <P> When an infection occurs, a chemical "SOS" signal is given off to attract phagocytes to the site . These chemical signals may include proteins from invading bacteria, clotting system peptides, complement products, and cytokines that have been given off by macrophages located in the tissue near the infection site . Another group of chemical attractants are cytokines that recruit neutrophils and monocytes from the blood . </P>

The first phagocytes to migrate to a bacterial infection are