In the 300 million years since the divergence of the avian and mammalian lines, numerous differences have evolved between the antibody responses of birds and mammals. The most significant of these differences results from the use of the bursa of Fabricius by birds. This unique organ serves as a site where B cells, the cells that will eventually produce antibodies, are first selected for their ability to produce antibodies against foreign antigens. This selection process is preceded by a relatively short period when the developing B cells generate a wide array of antibody receptor molecules by gene conversion. Thus, unlike mammals that can generate new antigen-binding specificities throughout their lives, birds can only do so during a short period within the bursa before hatching. A second key difference lies in the chemical structure of the major immunoglobulin (lg) involved. Thus in mammals, this is IgG. In birds it is now recognized as a distinctly different immunoglobulin called IgY. In addition, some birds can produce a small-version of the IgY that lacks a full-sized Fc region. Its biological significance is unclear. Finally, birds lay eggs and thus must pass maternal immunoglobulins on to their offspring within the egg contents. For this reason, the egg yolk is full of IgY, whereas the albumin is rich in IgA. Pigeons are a notable exception to this situation when they secrete a "crop milk" rich in IgA. Notwithstanding these differences, it must be emphasized that the biological role of antibodies is identical in all species tested to free the body of extracellular invaders. For this reason, the basic kinetics and function of the antibody response is well conserved between the different vertebrate classes. Copyright 2002 by W.B. Saunders Company.
