The close resemblance of a foreign molecule to one present naturally in the body. For example, opiates such as morphine exert their pain-relieving effects by mimicking the body’s natural endorphins, which are released to counter stresses such as strenuous exercise. Antigenic molecules of certain invading pathogens can mimic a host tissue molecule. This can cause the host’s immune system to produce antipathogen antibodies that will cross-react against self tissue, leading to autoimmune disease. For example, in some forms of rheumatic fever, the infecting streptococcal bacteria induce the formation of antibodies and even T cells that attack not only the bacteria but also various host tissues, including the heart valves, leading to serious long-term consequences.