The metabolism of the two subclasses of IgA was investigated in fourteen turnover studies in individuals with normal immunoglobulin serum concentrations. Differences in the catabolism of radioiodinated IgA1 and IgA2 myeloma proteins were found. The average biologic half-life was 5·9 days for IgA1 and only 4·5 days for IgA2. Fractional catabolic rates were 24% of the intravascular pool per day for IgA1 and 34% for IgA2. The intravascular compartment contained approximately 55% of the total body pools of both IgA subclasses.
The mean IgA1 serum concentration in the individuals used for turnover studies was 2·3 mg/ml, the mean IgA2 level was 0·32 mg/ml. Synthetic rates of 24 mg/kg per day for IgA1 and 4·3 mg/kg per day for IgA2 were calculated. The low serum concentration of IgA2 results mainly from this low synthetic rate. The fractional catabolic rate of IgA2 is higher than that of IgA1. This further increases the difference between the serum concentrations of the two IgA subclasses.
Quantitative determinations of both IgA subclasses were made in twenty-four sera of young male adults. The mean concentrations were 1·81 mg/ml for IgA1 and 0·22 mg/ml for IgA2.