Previous experiments displayed consistent individual behavioural differences in pigs. Some showed a more active behavioural response (aggressive and resistant; so-called A/R pigs), other a more passive behavioural response (non-aggressive and non-resistant; so-called NA/NR pigs). Moreover, these behavioural coping strategies were associated with different behavioural, physiological and endocrine responses under stress conditions. In the present study we selected 32 A/R and 32 NA/NR individuals and tested their immune reactivity in reaction to stress using several cell-mediated (CMI) and humoral immunological tests. Active A/R pigs had a higher in vivo and in vitro CMI to nonspecific and specific antigens, while after stress CMI reduced more in A/R than in NA/NR pigs. In contrast, humoral immunity was highest in NA/NR pigs. Furthermore, some serologically typed swine lymphocyte antigen (SLA) class I haplotypes were not equally distributed between A/R and NA/NR pigs. In general, these findings show that measurement of immune reactivity is an important tool to define how animals cope with environmental demands.