Plasma donation three times every 2 weeks resulted in significantly lower total serum protein and immunoglobulin G (IgG) concentrations compared to whole blood donation in the per-protocol analysis after 16 weeks, with mean differences (95% confidence interval) of −7.1 (−9.1, −5.0) g/L and −3.2 (−4.2, −2.2) g/L, respectively. IgG subclasses, other immunoglobulins, ferritin, haemoglobin and albumin were significantly reduced after 16 weeks.