Vaccination with a new vaccine against bovine viral diarrhea (BVD) caused unwanted immune reactions through alloreactive antibody production in 5–10% of vaccinated cows. Transfer of these alloreactive antibodies via colostrum caused highly lethal (90%) bovine neonatal pancytopenia (BNP) in calves. Despite the assumption of an immune deviant lymphocyte (PBL) phenotype, the exact immunological mechanisms driving these unwanted immune reactions are not fully understood to date. To gain deeper insight into these mechanisms, we analyzed the PBL secretome from alloreactive antibody producers (BNP cows) and non-alloreactive responders (controls) after stimulation with pokeweed mitogen (PWM).