The marginal zone is the region at the interface between the non-lymphoid red pulp and the lymphoid white-pulp of the spleen. (Some sources consider it to be the part of red pulp which borders on the white pulp, while other sources consider it to be neither red pulp nor white pulp.) A marginal zone also exists in lymph nodes. Structure It is composed of cells derived primarily from the myeloid compartment of bone marrow differentiation. More recently, a population of neutrophil-killers has been described to populate peripheral areas of the marginal zone. At least three distinct cellular markers can be used to identify cells of the marginal zone, MOMA-1, ERTR-9 and MARCO. Blood flow The marginal zone (MZ) is a highly transited area that receives large amounts of blood from the general circulation. Remarkably, the splenic microvasculature shows striking differences in mice and humans. In humans, the spleen receives blood from the splenic artery, which branches into central and penicillar arterioles. Owing to the absence of a histologically defined marginal sinus, the blood flowing in penicillar arterioles directly drains into capillaries of the red pulp and perifollicular zone. The perifollicular zone is a well-defined area of decreased resistance that separates the MZ from the red pulp. Both the perifollicular zone and the red pulp consist of an open circulatory system of blood-filled spaces known as splenic cords, which have no defined endothelial delimitation and are in close contact with the venous sinusoidal vessels of the red pulp. Function The major role of marginal zone is to trap particulate antigen from the circulation and present the antigen to the lymphocytes of the spleen. Experiments have shown that inert latex beads as well as live bacteria such as Escherichia coli and Listeria monocytogenes are trapped by the marginal zone. However, only immunogenic substances, i.e. bacteria, are trafficked to the T and B cell zones of the white-pulp and are efficiently presented to elicit an immune response. Lymphocytes Marginal zone lymphocytes are a type of B cell (Marginal-zone B cell, abbreviated "MZ B cell") created there, capable of binding IgM-antigen complexes. They are notable for their ability to serve several different roles in the immune system. MZ B cells express polyreactive BCRs that bind to multiple microbial molecular patterns. Marginal Zone Macrophages Within the marginal zone there exists two types of macrophages that are unique to this area: the marginal zone macrophages and