This 419 x 480 pixel image shows how a computer simulation tracked levels of killer T cells in a hypothetical patient. Initially (top) the T cells responded to a homogeneous population of HIV particles, each of which carried seven recognizable epitopes; epitope 5 elicited the strongest response (yellow). After a viral mutant carrying an altered, unrecognized version of this epitope emerged (middle panel), the dominant response became focused on a less stimulatory epitope--number 2 (red). And after epitope 2 mutated (bottom), dominance shifted again, to number 4 (green), an even weaker epitope. Such shifts could contribute to reduced immunologic control in HIV-infected patients.
