Patients with autoimmune polyendocrinopathy syndrome type 1 (APS-1) caused by autosomal recessive AIRE deficiency produce autoantibodies that neutralize type I interferons (IFNs) 1,2 , conferring a predisposition to life-threatening COVID-19 pneumonia 3 . Here we report that patients with autosomal recessive NIK or RELB deficiency, or a specific type of autosomal-dominant NF-kappa B2 deficiency, also have neutralizing autoantibodies against type I IFNs and are at higher risk of getting life-threatening COVID-19 pneumonia. In patients with autosomal-dominant NF-kappa B2 deficiency, these autoantibodies are found only in individuals who are heterozygous for variants associated with both transcription (p52 activity) loss of function (LOF) due to impaired p100 processing to generate p52, and regulatory (I kappa B delta activity) gain of function (GOF) due to the accumulation of unprocessed p100, therefore increasing the inhibitory activity of I kappa B delta (hereafter, p52LOF/I kappa B delta GOF). By contrast, neutralizing autoantibodies against type I IFNs are not found in individuals who are heterozygous for NFKB2 variants causing haploinsufficiency of p100 and p52 (hereafter, p52LOF/I kappa B delta LOF) or gain-of-function of p52 (hereafter, p52GOF/I kappa B delta LOF). In contrast to patients with APS-1, patients with disorders of NIK, RELB or NF-kappa B2 have very few tissue-specific autoantibodies. However, their thymuses have an abnormal structure, with few AIRE-expressing medullary thymic epithelial cells. Human inborn errors of the alternative NF-kappa B pathway impair the development of AIRE-expressing medullary thymic epithelial cells, thereby underlying the production of autoantibodies against type I IFNs and predisposition to viral diseases.|Inborn errors of the alternative NF-kappa B pathway in humans impair the development of AIRE-expressing medullary thymic epithelial cells, thereby underlying the production of autoantibodies against type I IFNs and predisposition to viral diseases