Chickens lack encapsulated lymph nodes like those of mammals and instead develop diffuse lymphoid tissue, wherever they are antigenically stimulated in the body. Since mucosae are primary targets for antigens, chickens have extensive mucosa-associated lymphoid tissues. These include the GALT dealt with here and in Chapter 13, the respiratory-associated lymphoid tissue, described in Chapter 14, and the reproductive-associated lymphoid tissue, described in Chapter 15.