Causal analysis of the variability of IgA, IgG, and IgM immunoglobulin levels

The interindividual variability of IgA, Igc, and IgM immunoglobulin levels was studied using path analysis in a northeastern Brazilian sample (nuclear families) to determine the genetic and/or environmental causes of their va riation. The path analysis model decomposes the phenotype into genetic caus es (autosomal and X-chromosome-linked genes) and environmental causes. A si gnificant familial aggregation, mainly resulting from autosomal components, was detected for the 3 immunoglobulin levels. The values of genetic herita bility were h(2) = 0.410 +/- 0.030 for IgA, h(2) = 0.617 +/- 0.020 for IgG, and h(2) = 0.540 +/- 0.023 for IgM, and the values for environmental-cultu ral heritability were c(2) = 0.085 +/- 0.034 for IgA, c(2) = 0.084 +/- 0.02 7 for IgG, and c(2) = 0.023 + 0.023 for IgM. Our results did not show a her itable component resulting from X-chromosome-linked genes on IgM levels, as suggested by some studies (Wood et al. 1969; Grundbacher 1972; Purtilo and Sullivan 1979). Some additional results were that (1) age and IgA concentr ation were positively correlated, with IgA level increasing gradually from childhood to adulthood (p < 0.001); (2) sex and the age x sex interaction a ct on IgG concentration (p < 0.01); (3) age and IgM concentration are corre lated (with children presenting lower levels than adults, especially in mal es, p < 0.01); and (4) a significant association exists between sex and IgM level (with females presenting higher levels than males, p < 0.001).