Ig_5 is an immunoglobulin domain found on T-cell surface glycoprotein CD3 epsilon chain. It forms a first-order complex with T-cell surface glycoprotein CD3 delta chain as part of the T-cell receptor complex [1].

1: Crystal structure of a human CD3-epsilon/delta dimer in complex with a UCHT1 single-chain antibody fragment. Arnett KL, Harrison SC, Wiley DC; Proc Natl Acad Sci U S A. 2004;101:16268-16273. PMID:15534202