Abstract: Abstract: Extant jawless vertebrates, represented by lampreys and hagfishes, have innate immune receptors with variable domains structurally resembling T/B-cell receptors. However, they lack cardinal elements of adaptive immunity shared by all jawed vertebrates: T/B-cell receptors and major histocompati?bility complex molecules. Thus, it was widely believed that adaptive immu?nity is unique to jawed vertebrates. Recently, this belief was overturned by the discovery of agnathan antigen receptors named variable lymphocyte receptors. These receptors generate diver?sity in their antigen-binding sites through assembling highly diverse leucine-rich repeat modules. The crystal structures of hagfish variable lymphocyte receptor monomers indicate that they adopt a horseshoe-shaped structure and likely bind antigens through the hypervariable concave surface. Secreted variable lymphocyte receptors form pentamers or tetramers of dimers bind antigens with high specificity and avidity. The fact indicates that jawed and jawless vertebrates have developed antigen receptors independently. This paper provides valuable clues for studying of the origin and evolution of adaptive immunity system.