Interleukin-4 induced 1 protein (IL4I1), a secreted amino acid oxidase produced by antigen presenting cells, oxidizes phenylalanine to phenylpyruvate. It has been found that IL4I1 exerts an immunosuppressive function by inhibiting the proliferation and differentiation of T cells as well as limiting the proliferation of B cells. IL4I1 is involved in host defense against infection. As a gene related to poor prognosis in cancers, IL4I1 participates in tumor immune escape. IL4I1 promotes remyelination via regulation of the different phenotypes of microglia in the autoimmune demyelinating diseases, but the detailed mechanism still remains unknown. We summarize the role and mechanism of IL4I1 in the immune regulation to provide new ideas for the treatment of infections, cancers and autoimmune diseases.