They are often the first to recognize infection and respond by mobilizing other cells of the immune system. This process is nearly identical for macrophages and neutrophils. B lymphocytes produce antibodies . Helper T lymphocytes secrete cytokines that augment several different immune system responses. 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.