Table 1.

Basic comparison of the adaptive immune system between avian and mammals

Vertebrate class	Immunoglobulin isotype
Mammals	IgM	IgD	IgG	IgE	IgA
Avian species	IgM		IgY		IgA

The adaptive immune system of avian and mammals is based on immunoglobulins. Birds produce three types of immunoglobulins (IgM, IgY and IgA), and the mammals five (IgM, IgD, IgG, IgE and IgA). In both cases the acquired immunity includes T-cell receptors (TCRs), polymorphic MHC class I and II molecules, primary and secondary lymphoid organs, rearrangement of the recombination- activating gene (RAG) and antibody class switch