Trauma causes the release of histamines by basophils and mast cells. Histamines cause vasodialation and increased vascular permeability. Injuries that cause bleeding result in immediate initiation of blood clotting. Macrophages at the site of injury or infection detect microbial antigens and products of dying cells and respond by releasing cytokines . White blood cells, especially neutrophils, chemotax to the site and become actively phagocytic.
Bacterial infections are particularly good at causing pus formation.
A mild or moderate fever is probably benefical to the host. The complement system consists of about 30 proteins present in blood. These proteins cooperate with each other to attack foreign cells that have invaded the body. Complement proteins alert cells of the immune system that an infection has occurred. They signal the immune system to mount an attack.