THE PHYSICS OF THE IMMUNE SYSTEM. When antigens (viruses, poisons, etc.) invade the body, the vigilant immune system first senses the danger and then produces an appropriate response. To do this, the defenders, a fleet of lymphocytes possessing as many as 10¹¹ molecular receptors, must perform a vast program of pattern recognition. A theory from the 1970's proposed that this process could be compared to a self-regulating multiply-connected network of agents. Physicists in Brazil have now taken the next step by simulating the immune performance with a system of cellular automatons, and have successfully modeled the actual behavior of the mouse immune system. Rita Maria Zorzenon dos Santos of the Universidade Federal Fluinense (011-55-21-620-6735, zorzenon@if.uff.br) and Americo Bernardes of the Universidade Federal de Ouro Preto have even been able to simulate correctly the effects of aging on the immune response. At the heart of their model is a "shape space" of possible receptor attributes including, for example, electrical charge, receptor geometry, and degree of activation. Zorzenon dos Santos believes that work on the immune system might offer insights into the behavior of complicated physics systems operating at far from equilibrium conditions. The next step for her immune research is the attempt to model the evolution of HIV infection and to study the way in which lymphocytes are activated in response to malaria. (Physical Review Letters, 5 October 1998.)