Mannose-binding lectin (MBL) serum level, C4 deposition, and MBL pathway activity. Functional MBL activity measured by three assays in all patients with systemic lupus erythematosus (SLE; n = 103), a subgroup of SLE patients without infections (SLEno; n = 52), a subgroup of patients with SLE who had minor infections only (SLEmin; n = 28), a subgroup of SLE patients with one or more major infections (SLEmaj; n = 23), and healthy controls. (a) MBL serum levels. (b) Complement C4 deposition. (c) MBL pathway activity. Each datapoint represents one patient. Bars show the median values. No significant differences were found between the median functional MBL activity of all patients with SLE and each of the subgroups or between the subgroups, as measured with three assays (data not shown).