Antigens are foreign substances, usually proteins or polysaccharides, which induce an immune response that releases antibodies. Pathogens, such as bacteria and viruses, are adept at changing their antigenic structure through gene mutation, thereby evading antibodies and escaping destruction. In addition, bacteria may disguise their antigens by coating themselves with the host's antibody molecules to hide their antigenic markers from the immune system.
Some types of bacteria emit antigens into surrounding tissues, creating a substance called an endotoxin that absorbs antibodies and renders them ineffective. The bacteria that cause the flu, whooping cough and cholera produce endotoxins. Endotoxins are part of the outer membrane of the bacterial cell. Conversely, while growing, bacteria emit small amounts of endotoxins that may actually stimulate immunity.
Bacteria can often trick the host into thinking that its antigens are similar to the host's, thereby causing immune suppression. Antibodies cannot distinguish between host and pathogen antigens and are unable to produce an immune response. In addition, when high doses of circulating bacterial antigens are present in the bloodstream, antibodies become acclimated to the bacteria and are not as diligent about responding to them. The opposite of tolerance occurs when antibodies over-react, thereby producing an autoimmune disease, causing the body to attack its own tissues.
Some bacteria carry antigens that attract ineffective or poorly-binding antibodies. Avidity, or the ability of an antibody to bind to an antigen, affects the antibacterial action of the antibody. Bacteria may direct antibodies to bind against antigens that are unimportant, thus weakening the antibody's effectiveness. Bacteria may also develop antigens that target weak-binding sites on antibodies, which not only prevents the antibody from disabling the bacteria, but also protects the bacteria from interacting with stronger-binding antibodies.
Some pathogens avoid detection from antibodies by hiding within the antibody molecule or within infected cells. Macrophages, which are immune cells that "eat" pathogens, often serve as a growth medium for bacteria, while also protecting them from the immune response. The mycobacteria are a type of bacteria that cause leprosy and tuberculosis; they contribute to the chronic nature of these diseases by hiding within macrophages.