By means of the indirect immunofluorescent method the occurrence of smooth-muscle antibodies (SMA) was studied in 582 normal subjects of different ages, and the incidence of SMA was compared with that of anti-nuclear antibodies (ANA). SMA were detected in 32 (5-5%) and ANA in 28 (4-8%) of the normal subjects. The antibodies were of the IgG and the IgM class, while no IgA antibodies could be detected. IgM-SMA were demonstrated in 2-9%, IgM-ANA in 4-1% and IgG-ANA in 1-2% of the 582 subjects, and the incidence of these antibodies increased with increasing age (P = 0-012, P less than 0-001 and P = 0-003, respectively). IgG-SMA occurred in 3-6% and the prevalence of these antibodies was not age-related. No relationship between the incidence of antibodies and sex could be established. IgM-SMA and IgM-ANA were often found together in the same individuals (P = 0-002), which suggests that the same factors may elicit the formation of both antibodies
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