Dynamics of the systemic components of the chicken (Gallus gallus domesticus) immune system following activation by Escherichia coli; implications for the costs of immunity

The immune response is thought to be costly and deters from growth and reproduction, but the magnitude and sources of these costs are unknown. Thus, we quantified the changes in mass of leukocytes (CD4+ and CD8+ T cells, Bu1+ IgM+ and Bu1+ IgG+ B cells, monocytes/macrophages, heterophils and thrombocytes) and protective plasma proteins in systemic (non-mucosal) components of adult chickens injected intravenously with dead Escherichia coli. During the first day after E. coli injection most types of blood leukocytes decreased and α-1-acid glycoprotein increased. Specific IgM, specific IgY, total IgM, Bu1+ lymphocytes in the spleen and bone marrow and thymic CD8+ lymphocytes increased at 5 d post-injection. Quantitatively, the increases in the weight of cells and antibodies due to E. coli were dwarfed by the increase in the weight of the liver and acute phase proteins. Thus the acute phase response was markedly more costly than the subsequent adaptive response. The weight of the cells and proteins of the systemic immune system prior to challenge was 0.14% of body weight. Following E. coli injection, the additional weight of the immune components and the hypertrophy of the liver resulted in a 3.6-fold increase in weight which is equivalent to 18.5% of a large egg.

► The mass of cells and defensive proteins in the systemic immune system was quantified.
► Cells and proteins of the systemic immune system were 0.14% of body weight.
► An E. coli challenge caused a 66% increase in the mass of leukocytes and proteins.
► Hepatic growth for acute phase protein secretion dwarfed the expansion of leukocytes.
► The summed mass (liver + cells + proteins) is equivalent to 18.5% of a large egg.