The frequency of selective deficiency of serum IgA was determined in a population of 64,588 new Finnish blood donors by Ouchterlony's double diffusion with 10 mug/ml as the limit of detection. The incidence was 1:396. Those found IgA-deficient were retested by hemagglutination inhibition and by radioimmunoassay. The calculated incidences of IgA levels below 0.5 and 0.015 mug/ml were 1:500 and 1:800, respectively. Statistically significant compensatory elevation of serum IgG was observed in the IgA-deficient donors. The IgM levels were not changed. Among 9,920 hospital patients, the incidence of IgA deficiency was 1:660. The age structure of the IgA-deficient patients was similar to that of the IgA-deficient healthy blood donors but lower than that of hospital patients in general. No difference was observed between the clinical history of IgA-deficienct blood donors and of the controls.

Serum IgA levels of 35 healthy IgA-deficient index cases, of their 180 first-degree relatives, and of 31 other family members were studied. IgA deficiency was detected in 7.2% of the first-degree relatives, which is significantly more than the 0.25% frequency of IgA deficiency in healthy Finnish blood donors. Subnormal serum IgA levels were found in additional 13 (7.2%) first-degree relatives. The familial clustering of IgA deficiency seemed to be controlled by multigenic factors. IgA-deficient persons and their family members were tested for antinuclear and antithyroid antibodies, for rheumatoid factor (by latex test) and for other anti-IgG antibodies (by Ripley test). Antibodies against bovine milk were also studied. The relatives of IgA-deficient persons did not have more of these antibodies than the controls. It is suggested that various serological abnormalities are a consequence of the IgA deficiency rather than of genetic defects per se.

Immunological abnormalities of 163 IgA-deficient blood donors were studied. Anti-IgA antibodies were found in 15, anti-IgG antibodies (measured by the Ripley test) in 34, latex test positivity in 21, anti-IgM antibodies in 9 and antibodies against bovine milk in 41 sera. These numbers differed significantly from the control groups of normal blood donors. In the studies of thyroglobulin antibodies, antinuclear antibodies and blood group A and B isoagglutinins there was no difference from the controls. Interrelationships of all findings were studies by statistical analysis. Correlations were found between latex and Ripley tests and rheumatic disorders as well as between high age and anti-IgA antibodies. Male subjects had antibodies against bovine milk more often than females. Otherwise no striking findings were brought up by the statistical analysis.

A follow-up study of 204 healthy blood donors with IgA deficiency, identified between 1971 and 1980, was carried out. Sera were initially screened by a double diffusion method and 182 were retested by a more sensitive haemagglutination inhibition method. A reexamination was performed in 1990 and, again, in 1991-1992 using an enzyme immunoassay (EIA) developed for the measurement of very low concentrations of IgA. The median follow-up period was 19 years, and in 159 (78%) subjects no serum IgA could be detected in any of the measurements. In 42 (21%) subjects, serum IgA was detectable (> 0.18 mg/L), but the level was below the lower limit of the reference range for adults (800 mg/L) and remained relatively constant. Three subjects showed minute amounts of IgA by EIA (0.2-3 mg/L) in one of the follow-up samples in 1990-1992, but the level was below the detection limit of the EIA (0.05 mg/L) in the other sample. Thus, not only does primary IgA deficiency appear to be permanent, but also lower than normal serum IgA levels remain the same in healthy adults.