Immunological tolerance. Dissociation between in vivo and in vitro reactivity in parabiosed mice.

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 F(1) 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 F(1) 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.