When a monocyte enters tissues, it differentiates into a macrophage. Macrophages can be found throughout the body. They can be considered the sentinel cells of the immune system. 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 .