High determinant density may explain the phenomenon of alloreactivity.

When two members of the same species differfrom each other in their MHC-encoded antigens, mixing their lymphocytes leads to a vigorous T-cell response. This is the phenomenon of alloreactivity, exempl here as a mixed lymphocyte response. In this article Michael Bevan presents a hypothesis to account for alloreactivity, based on the notion that the alloreactive target determinant is a sequence or conformational determinant contributed entirely by the foreign MHC molecule in isolation, which is expressed at high density on the antigen-presenting cell.