Chemical Analog
Another common evasion is a chemical analog of a disguise a spy might use behind enemy lines. The external chemistry of some bacte- ria and some worms is so similar to that of human cells that the host may have difficulty in recognizing them as foreign. (Thus antibodies sometimes attack both invader and host cells.) The streptococcus bacterium, a longtime associate of humans, is especially adept at this trick. The antibodies to some strains cause rheumatic fever, in which a person’s antibodies attack his or her own joints and heart. Similar antibody attack on nerve cells in the basal ganglia of the brain can cause Sydenham’s chorea, with its characteristic uncontrollable muscle twitches. Interestingly, many patients who have obsessive- compulsive disorder, a psychiatric illness characterized by excessive
hand washing and fear of accidentally harming others, had Syden- ham’s chorea in childhood. There is now growing evidence that the brain areas involved in obsessive-compulsive disorder are very close to those damaged by Sydenham’s chorea. Thus, some cases of obsessive- compulsive disorder may result from the arms race between the streptococcus and the immune system.
