Ruslan Medzhitov's studies of the innate immune system focus on Toll receptors. These receptors are involved in the recognition of pathogens and are evolutionarily conserved in insects, mammals, and plants. Medzhitov's group is looking at the molecular and cellular mechanisms that mediate the control of adaptive immune responses by Tolls and other receptors of the innate immune system. The innate immune system is evolutionarily ancient and is found in one form or another in all metazoans.
Adaptive immunity is present only in vertebrates.
The two systems use fundamentally different mechanisms to recognize invading pathogens. Innate immune receptors recognize conserved, invariant markers of pathogens that are chemically distinct from anything in the host. Therefore, innate immune receptors can discriminate between self and nonself and are specifically activated only when a pathogen is present. These antigen receptors are generated by random somatic gene rearrangement and are expressed in a clonal fashion on lymphocytes. Because the specificity of these receptors is generated at random, the receptors of the adaptive immune system cannot distinguish self from nonself.