The immune system defends against infection by employing tiered defenses with increasing specificity. Pathogens are kept out of an organism by physical barriers. When these barriers are overcome, the innate immune system responds with a non-specific, rapid reaction. This initial defense is provided by phagocytes (cells of the immune system that destroy bacteria) and natural antibodies. Natural antibodies are proteins produced by the body's immune system without specific antigen stimulation. They can be found in blood plasma or secretions such as saliva or breast milk. Antibodies are used by the immune system to identify foreign cells and substances for destruction. Antibodies are produced by B lymphocytes. After being exposed to a foreign material, B lymphocytes divide rapidly, producing large numbers of antibody molecules. These antibodies are then released into the blood stream or other tissues where they bind to the foreign material and mark it for removal by other cells of the immune system or by complement. Complement is another component of the immune system that aids in the identification and elimination of invaders. It can also trigger inflammatory reactions that help fight off infections. Immune responses are divided into two categories: cellular and humoral. The humoral immune response is mediated by antibodies which interact with specific antigens expressed on the surface of pathogens. These antibodies opsonize the pathogen causing it to be recognized by macrophages and other immune cells.