Thrombocytopenia means a decreased number of platelets (also called thrombocytes). Platelets are produced by megakaryocyte cells in the bone marrow and are necessary to help blood clot. Anytime a blood vessel is damaged and begins leaking blood, the odd shaped and sticky platelets clump together to plug the leak and prevent ongoing blood loss. Without a sufficient number of platelets, we would quickly bleed to death.
Thrombocytopenia can occur in HIV-positive people for a variety of reasons. First, it is known that HIV can infect megakaryocytes, which means that HIV itself can cause this condition. Second, certain drugs used to treat HIV/AIDS that can damage the bone marrow (e.g., some nucleoside reverse transcriptase inhibitors) and certain AIDS-related conditions (e.g., lymphoma and MAC), can result in decreased platelet production. Third, antibodies produced by the immune system can target healthy platelets in the body (a condition called immune thrombocytopenic pupura [ITP]). These antibodies are known as autoantibodies, as they are attacking the "self", and signal the spleen to destroy and remove the platelets from the body.
A normal platelet count is between 150,000 and 400,000 platelets per cubic milliliter of blood. In severe cases of thrombocytopenia, the platelet count can be close to zero. In mild cases, the platelet count is between 100,000 and 150,000. If the platelet count falls below 30,000, the risk of uncontrolled bleeding is high, including blood leakage in the brain (cerebral hemorrhage) that can lead to a stroke.
Platelets also carry serotonin and L-tryptophan, two substances involved in sleep/wake cycles, appetite, and mood regulation.