Castleman's isease is a rare disorder characterized by non-cancerous growths (tumors) that may develop in the lymph node tissue at a single site or throughout the body. It involves hyperproliferation of certain B cells that often produce cytokines. There are several variants of Castleman's disease. In all cases, Castleman's disease is likely due to hypersecretion of the cytokine IL-6. In tumors that are positive for Kaposi's sarcoma-associated herpesvirus (KSHV) this is most likely due to expression of the a virus-encoded cytokine, vIL-6, while KSHV negative tumors appear to be the result of over-secretion of human IL-6.