So while some species produce their lymphocytes in from stomach GALT, others have progressed to producing blood cells in the better protected bone marrow regions. We also see the subdivision of T cells into different specialist functional ability. With a few dukeing it out in the ring and a large audience shouting encouragement. Reptiles have a very similar immun system structure. Several species of snakes and lizards have basic lymphnodes that might be capable of functioning as our own.
Birds are quite close to mammals with a highly developed vertebrate immune system.
They have all the immune system organs that we have, spleen, thymus, GALT, and bone marrow. Not all birds have lymphnodes though. Sea birds and shore waders have a lymph node network but birds such as chickens do not. Birds do have an additional organ that we do not, the bursa of fabricius. This organ develops in the embryo close to the intestines.