Background: The co-existence of autoimmunity and primary antibody deficiency in some individiuals is intriguing. The transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) gene (TNFRSF13B) has been implicated in both autoimmunity and primary antibody deficiency to varying extents in mice and humans. However, the phenotype described in mice with TNFRSF13B polymorphisms has not been entirely consistent with patients with similar orthologous polymorphisms. Objective: To further understand the relationship between TNFRSF13B variants and PAD and autoimmunity, we set out to determine the association of the two most common TNFRSF13B polymorphisms with autoimmunity and immunodeficiency, in patients with primary antibody deficiency and SLE. Method: We genotyped the C104R and A181E polymorphisms of TNFRSF13B in193 individuals and 144 controls from the Australian and New Zealand Antibody Deficiency Allele (ANZADA) Study, 107 patients from the Australian Point Mutation in Systemic Lupus Erythematosus (APOSLE) study, 169 patients with SLE from a European population, and 263 European controls. We were also able to determine TNFRSF13B genotypes for family members for nine of twelve pedigrees with primary antibody deficiency identified with TNFRSF13B variants. Results: The total number of TNFRSF13B variants in the primary antibody deficiency cohort was significantly higher than in the control group (p=0.0089; OR 9.481 [95% CI 1.218−73.81]). Similar results were obtained when patients with systemic lupus erythematosus were analysed. TNFRSF13B variants were strongly associated with SLE (p=0.0161, OR 3.316 [95% CI 1.245-8.836]). Familial analysis revealed incomplete penetrance of the TNFRSF13B variants. Conclusion: Taken together, the two most common TNFRSF13B variants are associated with primary antibody deficiency and systemic lupus erythematosus.