The specific autoimmune conditions were SLE (18.8%, 6/32 vs. 5.8%, 7/120 in C4A deficient patients and controls, respectively, OR = 3.75, 95%CI = 1.16–12.01, p = 0.031) and coeliac disease (12.5%, 4/32 vs. 0%, 0/120 in C4A deficient patients and controls, p = 0.002). Here, C4A is linked to celiac disease.