Proteins of this family play an important role in inducing non‐specific resistance against a broad range of viral infections. They also affect cell proliferation and modulate immune responses. Produced by peripheral blood leukocytes and lymphoblastoid cells, IFN-alpha is an acid stable molecule that signals through IFNalpha/betaR, which is also used by IFN‐beta. Both IFNs have similar anti‐viral activity and regulate expression of MHC class I antigens. IFN‐alpha contains four highly conserved cysteine residues which form two disulfide bonds, one of which is necessary for biological activity. Recombinant Human IFN‐alpha (2a) is a 19.3 kDa protein containing 166 amino acid residues.