Ig Isotypes and IgG subclasses of FVIII-binding antibodies in healthy individuals in relationship to age and sex. Plasma samples obtained from healthy individuals were analyzed for FVIII-binding antibodies of the Ig isotypes and IgG subclasses (A) IgG1, (B) IgG2, (C) IgG3, (D) IgG4, (E) IgA, and (F) IgM. Healthy individuals were equally distributed by age between 18 and 66 years for both men (▴; n = 300) and women (▿; n = 300); see also supplemental Figure 2. Plasma samples were diluted at least 1:20. Samples that did not give a positive signal at this minimum dilution were considered as ative (not detectable [ND]). The dotted line at a titer of 1:80 indicates the minimum titer required for proof of specificity. Titers of < 1:80 were too low to be confirmed for specificity.