The costs of exploiting an organism's immune function are expected to form the basis of many life–history trade–offs. However, there has been debate about whether such costs can be paid in energetic and nutritional terms. We addressed this question in a study of wintering, free–living, male great tits by injecting them with a novel, non–pathogenic antigen (sheep red blood cells) and measuring the changes in their basal metabolic rates and various condition indices subsequent to immune challenge. The experiment showed that activation of the immune system altered the metabolic activity and profile of immune cells in birds during the week subsequent to antigen injection: individuals mounting an immune response had nearly 9% higher basal metabolic rates, 8% lower plasma albumin levels and 37% higher heterophile–to–lymphocyte ratios (leucocytic stress indices) than sham–injected control birds. They also lost nearly 3% (0.5 g) of their body mass subsequent to the immune challenge. Individuals that mounted stronger antibody responses lost more mass during the immune challenge. These results suggest that energetic expenditures to immune response may have a non–trivial impact upon an individual's condition.