Given that ustekinumab (Stelara) is produced in murine SP2/0 cells, a cell line known to introduce α-gal during post-translational modification, we hypothesized that α-gal could explain some of these reactions.<h4>Methods and results</h4>We describe six patients with Crohn's disease who experienced acute reactions during ustekinumab infusion and were found to have IgE specific to α-gal. The gene discussed is IGHE; the disease is Crohn disease.