Immunology of Birds

Abstract The immune system of birds diverged from that of mammals over 200 million years ago, and although the basic structure and function is largely similar, the immune system of birds is generally more compact and less polymorphic than that of mammals. Birds possess only a single polymorphonuclear leucocyte, the heterophil, but have cellular thrombocytes that may act as phagocytes and antigen‐presenting cells. Recognition of pathogens through pattern recognition is very similar to mammalian species. Within the adaptive immune response, birds have considerably less polymorphism of major receptor families. The most studied avian species, the chicken, has a compact major histocompatibility complex expressing only two Class I and Class II alleles and evidence of coevolution with major pathogens. There are only three immunoglobulin classes (IgY, IgM and IgA) but three T‐cell receptor classes. Functionally, the system is similar to mammals with cellular (Th1), humoral (Th2), regulatory and Th17 all described in the chicken. Vaccination is a widely used tool in disease control in chickens and other domestic birds. Vaccination technology is particularly advanced and includes the use of drinking water, spray and in ovo vaccines to facilitate delivery to large numbers of animals. Key Concepts Birds possess a complex immune system which, although shares much of its structure and function with mammals, has differences that reflect 200 million years of divergent evolution. Most of our knowledge of avian immunology comes from the domestic chicken and the development of vaccines for use by the poultry industry. B lymphocytes are named after the Bursa of Fabricius, the primary lymphoid organ where they mature in birds. Birds lack structured peripheral or mesenteric lymph nodes. The immune system of birds is more compact than that of mammals, with less polymorphism of major receptor families. Birds have three immunoglobulin or antibody classes compared to five in mammals. Birds possess three T‐cell receptor types, one more than mammals. Pattern recognition receptors such as Toll‐like receptors are largely conserved between mammalian and avian species. Birds possess a single polymorphonuclear cell class, the heterophil, but their nucleated thrombocytes appear to play a key role in innate immunity. The compact ‘minimal’ major histocompatibility complex of the chicken gives rise to close association between MHC type and resistance to infection. The need to vaccinate up to 50 billion animals reared each year has driven novel vaccines and delivery systems in the poultry industry.