Background: Predominantly antibody deficiencies (PAD) are inborn errors of immunity, and associate with increased susceptibility to infections. Life-long immunoglobulin replacement therapy (IgRT) is introduced based on Ig-levels and the frequencies and severity of infections. The subgroup of PAD with immunoglobulin G subclass deficiency (IgGsd) is often less severe and the need of IgRT in these patients is unclear. Often many years pass before IgGsd is diagnosed, and repeated respiratory tract infections risk to confer lung tissue damage. The Swedish standard regimen in IgGsd is IgRT during 18 months followed by a period of discontinuation in order to evaluate the effect and restart it, if infections reoccur. It is a challenge to identify the subgroup of IgGsd-individuals in need of life-long treatment, and early reliable markers are warranted. 

Aim: The aim of this thesis was to identify clinical and laboratory factors predicting the need of long-term IgRT in IgGsd and evaluate its effects on immunological functions. Results: In paper I, the established clinical warning sign that is used to identify individuals with PAD in need of IgRT, was evaluated regarding the accuracy to identify IgGsd in a primary care register with data from 350,000 visits due to respiratory tract infections. The predictive value was low and according to medical records, a primary immunodeficiency was rarely suspected. In paper II, a lower frequency of homozygosity for the high affinity IgG receptor FcγRIIa H/H131 were found in IgGsd compared to controls. The finding supports the hypothesis that FcγR-polymorphisms may be of importance for susceptibility to infections in IgGsd and may have impact on the need of IgRT. In paper III, patient reported outcome showed that severe fatigue and low health related quality of life associated with the need of life-long IgRT. Systemic inflammation is known to associate with fatigue and in paper III and IV extensive immunophenotyping was performed when on and off IgRT. IgGsd patients were characterized by dysregulated plasma protein profiles enriched for factors associated with interleukin 10 signaling that were unaffected by IgRT. Among circulating immune cells, decreased proportions of activated B- and T cell subsets, as well as regulatory T cells, were found and partly restored during IgRT. Individuals needing life-long IgRT had a lower prevalence of protective pneumococcal specific IgG against 21 tested capsular serotypes. Co-morbidity with autoimmunity, atopy, lung disease and other factors such as higher age and occupation also associated to the need of IgRT. 

Conclusion: IgGsd patients were characterized by increased systemic inflammation, which was unaffected by IgRT. Decreased activation of lymphocyte subsets was partly restored during IgRT. Severe fatigue, protective IgG levels against a low number of pneumococcal serotypes, especially in conjunction with comorbidities were associated with the need for IgRT. Overall, no factor alone could predict the need of life-long IgRT.