MBL levels of rheumatoid arthritis patients from extended Icelandic families compared with their first-degree relatives. Patients and relatives were stratified first according to their rheumatoid factor (RF) status into RF-negative (a) and RF-positive (b) and then according to their cigarette smoking status into never smokers (c) and ever smokers (d). The box plots show the median values and interquartiles, and the 10th percentiles of MBL concentrations are shown by the bars. MBL levels between two groups are compared by the Mann-Whitney rank sum test. MBL, mannan-binding lectin; RA, rheumatoid arthritis.