Spleen
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Cross section of the spleen of a pigeon (HE): white pulp
The white pulp is mainly organized around the arteries. It comprises arterioles surrounded by a periarterial lymphoid sheath (PALS) (vagina periarterialis lymphatica), occasionally lymphoid follicles, and penicillar capillaries surrounded by a peri-ellipsoid lymphocyte sheath (vagina periellipsoidea).