Avian eggs present an ideal alternative antibody source to mammals as the immunoglobulin (IgY) in the chicken's blood is transported to the egg and accumulates in the egg yolk in large quantities. The existence of an immunoglobulin G (IgG)-like molecule in avian eggs, referred to as IgY, has been well documented, and extensive research has been carried out on its characterization, production and purification. Although it is the functional equivalent of mammalian IgG, the major serum antibody found in mammals IgY is structurally different, and has been found to exhibit several important differences when compared to mammalian antibodies, including its physicochemical properties and immunological capabilities. Recently, considerable research has focus seldom use of IgY as an alternative to mammalian antibodies for several applications, including immunotherapeutic applications, especially for the oral passive immunization against various bacteria and viruses. Much research has also been carried out on the use of IgY as a replacement for IgG in various immunodiagnostic and immunoaffinity purification purposes. The use of IgY offers several advantages over polyclonal antibodies produced in mammals, including providing a much more hygienic, cost efficient, convenient, humane and plentiful source of antigen-specific antibodies.