Spleen

Abstract

The spleen, the largest secondary lymphoid organ in the body, functions both as a blood filter and part of the immune system. Histologically, the spleen is comprised of three main components; the red pulp, the white pulp and the marginal zone. The primary functions of the spleen are largely localized to specific anatomic compartments. The splenic red pulp serves as a blood filter to remove effete erythrocytes and platelets from the blood. Red pulp macrophages also have a role in combating blood-borne infection. The white pulp and marginal zone are the primary sites of innate and adaptive immune responses. The marginal zone is at the interface of red and white pulp, and has a predominance of macrophages, dendritic cells, and B cells that play an important role in innate immunity as well as the capture and presentation of antigens to initiate the adaptive immune response. Abundant lymphocytes in the white pulp are distributed into T cell-rich peri-arteriolar lymphoid sheaths and B cell-rich follicles, which work cooperatively to develop adaptive immune responses. A complex interplay between innate and adaptive immune cells and mediators makes the spleen important in the development of effective immune responses, particularly against circulating pathogens. In performing histological and functional evaluations, it is important to consider the wide range of responses in the spleen as well as differences in responses and background findings that can occur in animals of different species, strains, ages, or physiological states.