It has been shown that many pigeon materials contain one or more of four pigeon proteins—pigeon γ-globulin (PGG), pigeon serum albumin (PSA), pigeon β-globulin (PBG) and a protein cross-reacting immunologically with PGG termed XPGG. It is thought that the ability of seemingly unrelated sources of pigeon material, e.g. pigeon droppings, serum, egg yolk and white to react with sera from cases of pigeon breeders' disease is due to the presence of PGG, PSA, PBG or XPGG in these materials.
Only in pigeon droppings were important antigens other than PGG, PSA, PBG and XPGG found and it is possible that pigeon droppings are the only `complete' source of antigens concerned with the disease unless specified conditions arise.