The avian spleen: a neglected organ

The functions of the little-studied avian spleen are reviewed and compared with those of its better known mammalian counterpart, which is generally larger in proportion to body size than in birds. A role in immunity similar to that in mammals is evident, but the organ's contribution to oxygen supply seems less extensive; splenic storage of erythrocytes, for example, is unrecorded for birds. The spleen is a principal organ of systemic immunity, and its importance in disease resistance is presumably accentuated by the scarcity of avian lymph nodes. The striking intraspecific variation in size partly reflects seasonal changes in spleen morphology and activity. Several explanations, principally based on changing oxygen demand, have been proposed previously for these periodical cycles. But seasonally small spleens could sometimes simply stem from a combination of (1) a cessation of active splenomegaly as seasonally patent infections recede, and (2) a seasonal lymphoid involution, occurring even if an individual has not recently responded to, and recovered from, an infection. Possible determinants for these and other processes are discussed from evolutionary and ecological perspectives. There is a pressing need for a thorough investigation of both hematological and immunological functions, using a phylogenetically and ecologically broad range of species, as well as modern histological and experimental techniques.