Figure 1: Development of heterologous immunity.

A) A naïve T cell pool encountering viral antigen will proliferate to form an effector response, and contract to the memory phase once the infection is under control. B) T cell receptor clones that cross-react with a different virus (orange TCR clone, purple virus) can mount a heterologous response to that virus. C) Upon exposure to alloantigen (red cell), T cell receptor clones (green) that cross-react to allo can mount a heterologous response to alloantigen. Note: allorecognition can be mediated through either direct or indirect antigen presentation, as described in the text. For simplicity, we only show direct presentation in this figure.