Introduction

The immune system constitutes the principal defence mechanism of an organism. Defence mechanisms exist in most organisms from protozoa to vertebrates. In protozoans, such as amoeba, a nucleus may be transplanted into an enucleated syngeneic strain and the recipient will survive, but if the nucleus is from disparate strains, then the chance the recipient has of survival falls with increasing disparity between the strains, i.e. a basic form of recognition of self and non-self exists. In invertebrates, a phagocytic system exists for engulfing foreign microorganisms; however, it is only in vertebrates that the refined and highly discriminating system based on the production of humoral antibodies exists (Manning & Turner, 1976). Almost all vertebrates are able to produce immunoglobulins having the two heavy and two light chains (H2L2) as their principal antibody, the only exception being in the Agnatha, where the lamprey antibody consists of heavy chains linked only by non-covalent bonds.

There are number of important characteristics of the avian immune system, discussed in this chapter, some of which are common to vertebrates in general and others that have not been found outside the class Aves. Most of this chapter is devoted to two topics, namely, the generation of avian antibodies and the nature and diversity of avian histocompatibility antigens. There are a huge number of different immunoglobulins generated to interact specifically with antigenic groups.