Dipeptidyl peptidase-4 (DPP4) or adenosine deaminase complexing protein 2 (ADCP 2) or T-cell activation antigen CD26 (EC 184.108.40.206.) is a serine exopeptidase belonging to the S9B protein family that cleaves X-proline dipeptides from the N-terminus of polypeptides, such as chemokines, neuropeptides, and peptide hormones. The enzyme is a type II transmembrane glycoprotein, expressed on the surface of many cell types, whose physiological functions are largely unknown. Protein dimerisation should be required for catalytic activity and glycosylation of the enzyme could impact on its physiological functions. The dimeric glycoprotein ADCP has been found linked to adenosine deaminase (ADA) whose relationship with lymphocyte maturation-differentiation is well-established. Since implicated in the regulation of the biological activity of hormones and chemokines, such as glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide, DPP4 inhibition offers a new potential therapeutic approach for type 2 diabetes mellitus, as monotherapy and adjunct therapy to other oral agents. The clinical use of presently available orally active inhibitors of DPP4, however, has been associated with side effects that have been in part attributed to the inhibition of related serine proteases, such as DPP8 and DPP9. Indeed, it is noteworthy that CD26 has a key role in immune regulation as a T cell activation molecule and in immune-mediated disorder. All-cause infections were increased after sitagliptin treatment. It is noteworthy that the effects of DPP4 inhibition on the immune system have not been extensively investigated. So far, only routine laboratory safety variables have been measured in published randomised controlled trials. The review summarises present knowledge in the field and suggests some potential directions of future research.