Survival in a hostile environment requires the ability to mount a protective immune response while avoiding a reaction of the immune system against the self. We propose that the migration and localization of antigen are the governing factors in immunologic responsiveness or unresponsiveness against infections, tumors, and self and against xenografts and allografts. This conclusion is based largely on studies of experimental viral infection¹–³ and of the small numbers of donor leukocytes found in the blood and tissues of human and animal recipients of organ allografts (microchimerism).⁴–⁶ Under both circumstances, an immune response can be construed as a . . .