Phagocytosis is a nonspecific defense mechanism in which various phagocytes engulf and destroy the microorganisms of disease. Phagocytes. Among the important phagocytes are the circulating white blood cells called neutrophils and monocytes. In the tissues, the monocytes are transformed into phagocytic cells called macrophages. The macrophages move through the tissues of the body performing phagocytosis and destroying parasites. They are part of thereticuloendothelial system. Phagocytes also initiate the processes of the immune system. The process of phagocytosis begins with attachment and ingestion of microbial particles (Figure 1 ) into a bubblelike organelle called a phagosome. Once inside the phagocyte, the phagosome containing the microorganism joins with a lysosome, which contributes enzymes. The fusion of phagosome and lysosome results in aphagolysosome. Microorganisms are destroyed within minutes, and the microbial debris is eliminated from the cell in the process of egestion. In the immune process, chemical portions of the microorganism called antigenic determinants are displayed on the surface of the phagocyte to stimulate the immune process. Figure 1 The process of phagocytosis, a type of nonspecific defense to disease. Phagocytosis is enhanced by products of the immune system called antibodies. These protein molecules bind to microorganisms and encourage engulfing by phagocytosis. The complement system. The complement system is a series of proteins that circulate in the blood and encourage phagocytosis or otherwise “complete” the defensive process. Many immune reactions stimulate the complement system. The complement system operates in a cascade of reactions. In the pathway, certain complement components react with one another and produce new substances that induce other components to react. The results of the myriad reactions are substances that induce other complement components into action. The overall result is a number of substances toxic to microorganisms. The substances encourage phagocytosis or bring about destruction of microbial membranes. Two general pathways for complement activity exist. The classical pathway operates with the highly specific immune system and is initiated when certain antibodies unite with antigens and stimulate the complement system into action. The alternative pathway is nonspecific and is initiated by tumors, cell wall components of bacteria, and various microorganisms. It is sometimes called the properdin pathway because properdin is one of the proteins operating in it. The alternative pathway invokes a slower and less specific method for ridding the body of parasites, particularly Gram-negative bacteria and viruses.