Two types of soluble TNF receptors (sTNFR-I and sTNFRII) have been identified, which act to neutralize the biological activities of TNF alpha and TNF beta. Levels of these soluble receptors appear to arise as a result of shedding of the extracellular domains of the membrane bound receptors. Elevated levels of soluble TNF receptors have been found in the amniotic fluid of pregnant women.