  The study of PT-symmetric physical systems began in 1998 as a complex
generalization of conventional quantum mechanics, but beginning in 2007
experiments began to be published in which the predicted PT phase transition
was clearly observed in classical rather than in quantum-mechanical systems.
This paper examines the PT phase transition in mathematical models of
antigen-antibody systems. A surprising conclusion that can be drawn from these
models is that a possible way to treat a serious disease in which the antigen
concentration is growing out of bounds (and the host will die) is to inject a
small dose of a second (different) antigen. In this case there are two possible
favorable outcomes. In the unbroken-PT-symmetric phase the disease becomes
chronic and is no longer lethal while in the appropriate broken-PT-symmetric
phase the concentration of lethal antigen goes to zero and the disease is
completely cured.
