IFN-ɣ is an acid-labile interferon produced by CD4 and CD8 T lymphocytes as well as activated NK cells. IFN-ɣ receptors are present in most immune cells, which respond to IFN-ɣ signaling by increasing the surface expression of class I MHC proteins. This promotes the presentation of antigen to T-helper (CD4+) cells. IFN-ɣ signaling in antigen-presenting cells and antigen-recognizing B and T lymphocytes regulate the antigen-specific phases of the immune response. Additionally, IFN-ɣ stimulates a number of lymphoid cell functions including the anti-microbial and anti-tumor responses of macrophages, NK cells and neutrophils. Human IFN-ɣ species-specific and is biologically active only in human and primate cells. Recombinant human IFN-ɣ is a 16.8 kDa protein containing 144 amino acid residues.