Abstract When a microbe enters the body, it or its products encounter tissue macrophages. If the macrophage recognizes the pathogen it is activated to release a range of factors. Tissue damage and damage to the endothelium, which often accompany infection, result in activation of the clotting, kinin, fibrinolytic and complement systems. Activation of these systems leads to the generation of factors with many biological functions, including vasodilatation, increased vascular permeability and activation of mast cells. Activated mast cells degranulate, releasing the contents of their granules into the surrounding tissue. The granules contain histamine, heparin and degradative enzymes. Cytokines released in the area increase the expression of adhesion molecules on the endothelium, and chemokines (chemotactic cytokines) attract neutrophils and monocytes to the site. The neutrophils and monocyte/macrophages attempt to phagocytose and kill the infectious microbes, and monocyte/macrophages also remove dead cells and damaged tissue. If the inflammatory response is large enough, cytokines produced by macrophages appear in the bloodstream in sufficient concentrations to affect other tissues and organs. The local inflammatory and acute-phase responses may be enough to resolve an infection. If not, the next response of the immune system is to generate a specific immune response.