Hens and roosters of two distinct lines differ in immune cell profiles.

Sexual dimorphism in immune function influences disease susceptibility, yet sex-specific immune characteristics in chickens remain poorly understood, particularly across genetically divergent lines. This study investigated sex- and line-dependent differences in the peripheral and lymphatic immune system in lines selected for high and low feather pecking. Immune cell subsets were quantified in blood, spleen and cecal tonsils, and functional immune parameters were analyzed by measuring plasma antibody concentration and mitogen-induced lymphocyte proliferation capacity. Distinct immunological differences were observed between roosters and hens, as well as between lines, with several effects being tissue- and sex-specific. The higher overall numbers of immune cells observed in the spleen of roosters were primarily attributable to their larger organ mass, whereas in the cecal tonsils this was mainly due to a higher cell density. Roosters had fewer peripheral B cells, lower antibody concentrations and B lymphocyte proliferation capacity, along with fewer CD4+ cells and a lower CD4+/CD8α+ ratio. However, they had more γδ T cells compared to hens, indicating a greater reliance on cytotoxic or innate immune mechanisms rather than on adaptive, B cell mediated immunity. Line-specific immune differences were also observed, with the number of CD4+ cells in the cecal tonsils being lower exclusively in high feather pecking hens, supporting the hypothesis that feather pecking might be an immune-related behavior, potentially based on immune imbalances in the gut. In conclusion, immune profiles are strongly influenced by both sex and genetic background. Our results further emphasize the importance of considering sex when analyzing immune responses, as well as in vaccination strategies, disease management, and breeding programs.