Macrophages can be activated to become more phagocytic and more lethal in response to several signals that indicate infection. LPS activates macrophages to become more lethal. Macrophages are activated in response to several other signal molecules made during infection. Activated macrophages secrete toxins that kill nearby cells. Activated macrophages display portions of the protein antigens that they have ingested and partially digested on their cell surface.
Neutrophils stay in the blood until they receive a signal that injury or infection has occurred.
They then migrate to the site of infection and begin phagocytosis and killing. Neutrophils contain granules of concentrated antimicrobial peptides and hydrolytic enzymes. They are potent killers. This process is facilitated by vasodialation . There are about 10,000 Mast cells per cubic millimeter of skin, and somewhat fewer in more internal tissues.