General description

In most birds, except for some aquatic species, typical lymph nodes do not occur. Instead, the mucosa associated lymphoid tissue, mainly located in the intestinal and respiratory tract, is very well developed. This lymphoid tissue is composed of scattered or aggregated lymphoid cells and at some locations can even be organized into lymphoid tonsils.

The gut-associated lymphoid tissue (GALT) comprises: a pharyngeal ‘tonsil’; diffuse lymphoid tissue, lymphoid follicles and a tonsil in the esophagus; diffuse lymphoid tissue in the proventriculus; a pyloric tonsil; Peyer's patches; Meckel's diverticulum; caecal tonsils; diffuse lymphoid tissue in the rectum and  proctodeum of the cloaca.