Introduction Immunoglobulin D (IgD) remains an enigmatic molecule due to the limited understanding of its function both in healthy and in patients with autoimmune diseases.

Objectives In this study we analysed serum IgD (sIgD) levels in patients with rheumatoid arthritis (RA) and healthy controls, paying special attention to differences related to age, gender, smoking and presence of autoantibodies

Methods sIgD levels were measured in 416 individuals: 248 (184 female, 65 male) RA patients randomly selected at Sahlgrenska and Uddevalla Hospitals, and 169 (95 female, 73 male) healthy controls selected from the OLIN epidemiological study. Sandwich ELISA was developed by using mouse monoclonal antibodies for capture and goat polyclonal for detection (both SouthernBiotech). Serum samples were tested in dilution 1:5000 and sIgD was quantified after serial dilution of human serum with known IgD levels (Siemens, OTRD). The detection limit was 0.08 µg/ml. Smoking information was collected from all RA patients and controls by self-reported questionnaires. ACPA and RF were measured by automatic multiplex method (anti-CCP2) and rate nephelometry technology. Statistical analysis was performed using the Mann-Whitney test.

Results Median sIgD was 38 µg/ml (IQR 14–97) in RA and 43 µg/ml (IQR 19–108) in healthy controls. sIgD was lower in RA females than RA males (p=0.039) whereas healthy controls had no gender differences in sIgD. Among the females, healthy controls<50 y had high sIgD compared to healthy >50 y (p=0.009), and to RA patients<50 y (p=0.014) and RA >50 y (p=0.046). sIgD was not related to the disease activity, however, RA females producing RF alone (p=0.009) and in combination with ACPA (p=0.028) had low sIgD compared to non-producers. In RA, female smokers had high sIgD compared to never (p=0.022) and former smokers (p=0.003). Smoking was not affecting sIgD in healthy controls. No consistent difference in sIgD was found in males.

Conclusions The present study indicates that sIgD is influenced by age, gender, smoking and presence of autoantibodies. Low sIgD levels seem to be pathological due to their association with RA and the presence of RF and indicate a potential link between serum IgD and disease severity.

Disclosure of interest None declared