gut associated lymphoid tissues (GALT), conjunctival associated lymphoid tissues (CALT) and lymphoid nodules. There is also a lymphatic circulatory system of vessels and capillaries that transport lymph fluid through the bird’s body and communicate with the blood supply (Panada et al., 2007; Davison et al., 2008).
Functionally, the bird’s immune system can be divided into two parts: one innate, but non-specific, while the other part is acquired and specific. The acquired immune system is characterized by specificity, heterogeneity, and memory. This system is divided into cellular branch and non-cellular (humoral) branch.