In pigs, physiological imbalance may lead to an increased motivation to explore and forage, thereby enhancing the risk of tail biting behaviour under suboptimal environmental conditions. In several species genetic and physiological relations have been found between damaging behaviours and immune responses. Therefore, we studied relationships between immune parameters and tail biting in pigs. Pigs (n=480) weaned at 4 weeks were housed either barren (B; concrete floor with slats) or enriched (E; straw-bedding). Tail biting and tail wounds were recorded during the weaner (WP), grower (GP), and finisher phase (FP). Per phase, pigs were characterized as tail biter, victim, tail biter/victim, or neutral. Leukocytes, immunoglobulins (Ig), complement activity and haptoglobin levels were determined at weeks 8, 9 (3 days after 24-h mixing) and 22. Mixed models with type of pig and round (1-5) as fixed effects and pen as random effect were used per phase and housing condition. Leukocyte counts were unrelated to tail biting. In several phases, however, tail biters had highest IgG titers (B: GP P<0.01; E: FP P<0.05), the highest increase from w8 to w9 in IgG (B: P<0.10) and in haptoglobin concentrations (B: P<0.01), and lowest haptoglobin concentrations in w8 (B+E: GP P<0.05), w9 and w22 (E: FP P<0.05 and 0.10, respectively). B biters in FP had, furthermore, highest classical complement activity in w8 (P<0.01). E biters in FP had lowest alternative complement activity in w8 with the highest reduction after mixing (P<0.05). Hence,tail biters showed differences in immune parameters from other pigs. However, relationships were not always consistent over phases and housing, suggesting a temporary physiological change in tail biters. To conclude, tail biting seems to be associated with the immune status of pigs. This knowledge may help in understanding the etiology of tail biting and to develop preventive strategies against damaging biting behaviours.