Although most mouse strain combinations succumb to a lethal wasting disease when parabiosed across a strong histocompatibility (H-2) barrier, a high proportion of DBA mice parabiosed to DBA/C3H F1 hybrids survive and appear healthy. DBA mice accept C3H skin grafts following parabiosis, and may therefore be considered operationally tolerant of C3H antigens. Nonetheless, spleen cells from long-term DBA and F1 parabionts give normal and enhanced responses, respectively, to C3H antigens in mixed lymphocyte culture (MLC). This indicates that the tolerance mechanism can distinguish between MLC recognition reactivity and in vivo effector reactivity, and that the former can therefore exist in the absence of the latter.

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