The immune system comprises a variety of components that cooperate to defend the host against infectious agents. These components generally can be divided into nonspecific (or native) immune defense mechanisms and specific (or acquired) immune defense mechanisms. The nonspecific defense mechanisms are not antigen specific. They are present in a normal animal without previous exposure to antigen, and they are capable of responding almost immediately to an infectious agent. The major components of the nonspecific immune system are complement, phagocytic cells (macrophages, neutrophils, and eosinophils), natural killer cells, and some types of interferon. These components are very important in controlling an infection during the first few days after an initial exposure to an agent. This is the time when the specific immune response system is gearing up to produce antibody and a cell-mediated immune response.