From BCG vaccination to multifocal osteomyelitis and abscess: a case report of IL12R$$\:\varvec{\beta\:}$$1 deficiency

Mendelian Susceptibility to Mycobacterial Disease (MSMD) arises from defects in the interferon-gamma (IFN- \\(\\:\\gamma\\:\\) ) immunity, and most commonly due to IL12 receptor \\(\\:{\\upbeta\\:}\\) 1 ( IL12R \\(\\:\\beta\\:\\) 1 ) deficiency. Patients have reduced IFN- \\(\\:\\gamma\\:\\) production, impaired immunity to intracellular pathogens, and a higher risk of infections with weakly virulent mycobacteria, including Bacillus Calmette–Guérin (BCG), non-tuberculous mycobacteria (NTM), and Salmonella. We report a 10-year-old female born to consanguineous parents who presented with recurrent lymphadenitis, extensive abscess formation and multifocal osteomyelitis due to BCG. The patient harbored a homozygous deleterious variant in IL12R \\(\\:\\beta\\:\\) 1 (c.517 C > T, p.Arg173Trp), confirming MSMD diagnosis. IL12R \\(\\:\\beta\\:\\) 1 deficiency should be considered in children with unusual or disseminated mycobacterial infections, particularly in the setting of consanguinity. Molecular confirmation enables accurate diagnosis, treatment, prognosis, and genetic counseling.