MBL2 and yellow nail syndrome: Undetectable MBL may be indicative of, or at least a marker for immunodeficiency syndromes in RA, which in turn predispose to sino-pulmonary infections as is the case for example in common variable immune deficiency (CVID) in association with RA and possibly also coexistent RA and yellow nail syndrome, in which the frequency of Br is reported to be up to 89% [25].