The complement system is a set of plasma proteins that enhance nonspecific and specific defenses. About twenty kinds of complement proteins circulate in the blood in inactive form. These proteins are activated in a cascading fashion when defenses are breached. Chemical gradients of proteins attract phagocytes to the scene. Inflammation The Roles of Macrophages and Their Kin White blood cells, produced from stem cells in bone marrow, not only circulate in blood and plasma, but also reside in lymph nodes, spleen, liver, kidneys, and so on, where they stand ready to defend.
Eosinophils secrete enzymes that punch holes in parasitic worms.
Basophils secrete histamine, which sustains inflammation. While complement proteins are being activated, basophils and mast cells secrete histamine, which promotes leakage of fluid out of capillaries. Localized warming and redness occur at the site of damage or invasion. Fluid seeps from blood vessels causing swelling and delivery of infection-fighting proteins to the tissues. Neutrophils and macrophages engulf foreign invaders and debris.