The immune system is the third line of defense. It consists of mechanisms and agents that target specific antigens (Ags). An antigen is any molecule, usually a protein or polysaccharide, that can be identified as foreign (nonself) or self (such as MHC antigens described below). It may be a toxin (injected into the blood by the sting of an insect, for example), a part of the protein coat of a virus, or a molecule unique to the plasma membranes of bacteria, protozoa, pollen, or other foreign cells. Once the foreign antigen is recognized, an agent is released that targets that specific antigen. In the process of mounting a successful defense, the immune system accomplishes five tasks: Recognition. The antigen or cell is recognized as nonself. To differentiate self from nonself, unique molecules on the plasma membrane of cells called the major histocompatibility complex (MHC) are used as a means of identification. Lymphocyte selection. The primary defending cells of the immune system are certain white blood cells called lymphocytes. The immune system potentially possesses billions of lymphocytes, each equipped to target a different antigen. When an antigen, or nonself cell, binds to a lymphocyte, the lymphocyte proliferates, producing numerous daughter cells, all identical copies of the parent cell. This process is called clonal selection because the lymphocyte to which the antigen effectively binds is “selected” and subsequently reproduces to make clones, or identical copies, of itself. Lymphocyte activation. The binding of an antigen or foreign cell to a lymphocyte may activate the lymphocyte and initiate proliferation. In most cases, however, a costimulator is required before proliferation begins. Costimulators may be chemicals or other cells. Destruction of the foreign substance. Lymphocytes and antibodies destroy or immobilize the foreign substance. Nonspecific defense mechanisms (phagocytes, NK cells) help eliminate the invader. Memorization. Long‐lived “memory” lymphocytes are produced and can quickly recognize and respond to future exposures to the antigen or foreign cell.