The significance of beta2-integrins for the generation of antigen-specific immune responses in vivo was studied employing the bovine model of beta2-integrin deficiency. To that end four cattle with bovine leukocyte adhesion deficiency (BLAD) and healthy age-matched controls were immunized with tetanus toxoid (TT) and rabies virus (RV) vaccines three times in monthly intervals. In addition, two animals with BLAD and three controls received a fourth vaccination 8 months after the start of the study. Proliferative responses of peripheral blood mononuclear cells (PBMC) to the antigens TT and RV as well as specific serum immunoglobulin G (IgG) titers were determined in intervals for up to 10 months after primary vaccination. Proliferative responses of PBMC to TT and RV were substantially lower in cattle with BLAD than in controls, although PBMC from cattle with BLAD were shown to have the capacity to proliferate in the response to the mitogen concanavalin A. Occurrence of antigen-specific IgG titers was delayed and they were considerably lower in cattle with BLAD compared to controls. Finally, treatment of TT- and RV-stimulated PBMC from an immunized control with different concentrations of the anti-CD18 monoclonal antibody R15.7 resulted in a dose-dependent inhibition of lymphocyte proliferation to almost 100%. The results of the present study show that beta2-integrin deficiency leads to delayedand severely impaired immune responsiveness in vivo. The observations that antibody production, although considerably delayed and impaired, does occur and that apparently class-switching takes place in BLAD indicate T-cell reactivity in vivo.