The spleen is an incredibly important organ in the hematopoietic system. Normally, the spleen stores one-third of the platelets that are produced by the bone marrow and the remaining two-thirds of the platelets produced by the bone marrow are in circulation. No platelets are stored in the bone marrow.
If a condition causes the spleen to enlarge (splenomegaly), the spleen will function abnormally, sequestering up to 90% of the total platelet mass in the spleen. The result is a decrease in circulating platelets (thrombocytopenia). Some conditions that are associated with splenomegaly and a resulting thrombocytopenia include hepatic cirrhosis, Gaucher's disease, some leukemias (like hairy cell leukemia), and lymphomas.