RESUMEN
BACKGROUND: X-linked agammaglobulinemia, a primary immunodeficiency, can present with musculoskeletal manifestations. CASE CHARACTERISTICS: A 4-year-old boy, diagnosed as systemic juvenile idiopathic arthritis at the age of 3 years and treated with biological agents, presented with fever, dyspnea and chest pain. Blood culture and pericardial fluid culture revealed Achromobacter xylosoxidans. OUTCOME: Investigation revealed normal serum ferritin but low levels of serum immunoglobulins. Further immunological work-up revealed diagnosis of X-linked agammaglobulinemia. Child improved on antibiotic therapy; treatment with steroids and biological was discontinued. MESSAGE: Underlying immunodeficiency disease must be looked for in children suspected to have juvenile arthritis, more so if they develop unusual serious infection in response to immunomodulatory therapy.