The splenic artery enters at the hilum and branches into numerous arterioles that enter the splenic compartments. The tissue within the splenic compartments is formed by a reticular framework that contains lymphocytes and macrophages arranged around the sinusoidal drainage system. The regions encircling the white pulp are called the marginal zones . Branches from the central arterioles, the penicullar arterioles terminate in sheathed capillaries that are open to the marginal zones and are surrounded by aggregates of macrophages. Spaces between the endothelial cells allow blood borne cells and antigens to have free access to the white pulp where the immune response to blood borne antigens are initiated by T cells, B cells, macrophages and APCs.
Blood then drains into the red pulp where the RBCs are screened by resident macrophages .
The red pulp is permeated by a network of splenic sinusoids that are lined by long thick endothelial cells held loosely together by hoop-like reticular fibers. The blood then drains to the splenic veins and hepatic portal system. Solitary nodules occur randomly along these tracts as either uniformly dense primary nodules or secondary nodules generated by antigen-activation. Clusters of nodules that constitute distinct lymphoid organs mark several sites along the alimentary tract. These MALTs lack afferent lymphatics , and the epithelium overlying the lymphoid tissue is specialized to allow the detection of foreign antigens in the contents of the lumen.