The spleen is the largest lymphoid organ in the body and sitated in the left hypochondrium. There are two anatomical components:
1 The red pulp, consisting of sinuses lined by endothelial macro phages and cords (spaces)
2 The white pulp, which has a structure similar to lymphoid follicles Blood enters via the splenic artery and is delivered to the red and white pulp. During the flow the blood is ‘skimmed’, with leucocytes and plasma preferentially passing to white pulp. Some red cells pass rapidly through into the venous system while others are held up in the red pulp.
SEQUESTRATION AND PHAGOCYTOSIS. Normal red cells, which are flexible, pass through the red pulp into the venous system without difficulty. Old or abnormal cells are damaged by the hypoxia, low glucose and low pH found in the sinuses of the red pulp and are therefore removed by phagocytosis along with other circulating foreign matter. Howell-Jolly and Heinz bodies and sideroblastic granules have their particles removed by ‘pitting’ and are then returned to the circulation. IgG-coated red cells are removed through their Fc receptors by macrophages.
EXTRAMEDULLARYHAEMOPOIESIS. Pluripotential stem cells are present in the spleen and proliferate during severe haematological stress, e.g. haemolytic anaemia, thalassaemia major.
IMMUNOLOGICAL FUNCTION. About 25% of the body’s T lymphocytes and 15% of B lymphocytes are present in the spleen. The spleen shares the function of production of antibodies with other lymphoid tissues.
BLOOD POOLING. Up to one-third of the platelets are sequestrated in the spleen and can be rapidly mobilized.
Enlarged spleens pool a significant percentage (up to 40%) of the red cell mass.