A person with cancer may have blood with an unusually low level of platelets. This condition is called thrombocytopenia.Platelets are made in the bone marrow. Bone marrow is the soft, spongy tissue found inside larger bones. Platelets, also called thrombocytes, stop bleeding by helping clot blood and plugging damaged blood vessels. Thrombocytopenia happens when:The body does not make enough platelets.