The disadvantage of this is that phagocytes will not engulf copycat invading cells with a protein marker that resembles the self-marker. The skin, digestive tract, and respiratory tract employ nonspecific defense mechanisms such as bacteriocidal secretions, production of acid and mucus, ciliary motion, and simple barriers to entrance into the body. Macrophages kill cells by engulfing the ones presenting the appropriate antigen. Natural killer cells kill not the invading pathogen, but rather, the infected cell, by punching holes in the cell membrane and allowing the infected cell to lyse. The complement system is composed of a series of about 20 freely-circulating proteins in the blood.
Water can then enter the cell, ultimately causing it to burst.
Macrophages activate helper T cells by engulfing the invading pathogen and presenting the pathogen's antigens on its surface for the helper T cells to recognize. Once activated, helper T cells secrete lymphokines that mobilize cytotoxic T cells to fight off the pathogen. IgM functions to attract and persuade the proteins of the complement system in for attack. IgG marks the invading pathogen for phagocytosis. A T receptor resembles one arm of the antibody molecule, with alpha and beta chains, that has both constant and variable regions.