The recently classified type III interferon group consists of three IFN-λ (lambda) molecules called IFN-λ1, IFN-λ2 and IFN-λ3 (also called IL29, IL28A and IL28B respectively). These IFNs signal through a receptor complex consisting of IL10R2 (also called CRF2-4) and IL28RA (also called IFNLR1, CRF2-12). Recently, a new protein with a similar function related to IFN-λ3 was found around the same genomic locus and was designated IFN-λ4. Its intracellular signaling was through IFNLR1 and therefore thought to be a type III interferon. However, the evidence of its in vivo bioactivity is still debatable.
Acceptance of this classification is less universal than that of type I and type II, and unlike the other two, it is not currently included in Medical Subject Headings.