Undetectable IgA (<7 mg/dL) and IgM (<6 mg/dL) serum levels at time of CVID diagnosis were more likely in patients with gastric cancer in comparison to those without that complication (IgA: OR 27.5, 95%CI: 1.5–475.9, p = 0.027; IgM: OR 7.4, 95% CI: 1.5–36.1, p = 0.013). The gene discussed is CD40LG; the disease is common variable immunodeficiency.