[2] Ig-domains possess a characteristic Ig-fold, which has a sandwich-like structure formed by two sheets of antiparallel beta strands.

Interactions between hydrophobic amino acids on the inner side of the sandwich and highly conserved disulfide bonds formed between cysteine residues in the B and F strands, stabilize the Ig-fold.

The Ig domain was reported to be the most populous family of proteins in the human genome with 765 members identified.

[5] Members of the family can be found even in the bodies of animals with a simple physiological structure such as poriferan sponges.

They have also been found in bacteria, where their presence is likely to be due to divergence from a shared ancestor of eukaryotic immunoglobulin superfamily domains.