Primary immunodeficiency with rheumatic manifestations — branching viva

Opening question

Take me through your framework when a child presents with a symmetric polyarthritis that coexists with recurrent infection, chronic diarrhoea, and autoimmune cytopenia. Why are autoimmunity and immunodeficiency two faces of one failing immune system, and what is the threat gate that earns an immune work-up? [3] [1]

Branch 1 — the likely diagnosis and the confirming test

The girl's arthritis is clinically indistinguishable from juvenile idiopathic arthritis, but she has recurrent pneumonia, chronic diarrhoea, thrombocytopenia, and a low total IgG. What is the most likely diagnosis, and which first-line tests confirm it? Why is the pneumococcal vaccine response decisive, and why does the B-cell count matter? [7] [8]

Branch 2 — the management hazard and the parallel therapy

Her arthritis and thrombocytopenia persist despite the team's plan for a tumour-necrosis-factor inhibitor. Why must the biologic be deferred until the immune screen is back? Why does immunoglobulin replacement not reliably control the autoimmune and granulomatous features of CVID, and what is the principle that should govern the management of both arms? [7] [3]

Branch 3 — the contrasting case (CGD colitis)

Now picture an eight-year-old boy with a granulomatous colitis indistinguishable from Crohn disease, plus a staphylococcal liver abscess and a Serratia pneumonia. What is the most likely underlying defect, and which single test confirms it? Why is treating this colitis with immunosuppression before that test dangerous, and what is the role of prophylaxis and haematopoietic stem cell transplantation? [4] [5]

Branch 4 — the second contrasting case (complement-deficient lupus)

Finally, a five-year-old boy has severe childhood lupus with nephritis, a family history of lupus and meningococcal disease, and a CH50 that is persistently absent when checked well. What is the diagnosis, what is the mechanism by which classical complement deficiency produces lupus, and why must the CH50 be checked when the child is well rather than during a flare? What additional prevention does this diagnosis require beyond standard lupus therapy? [9] [10]

Closing — the general rule

In one sentence, what is the single rule that decides whether to start immunosuppression in a child whose immune screen is pending, and what is the single rule that matches the confirmatory test to the immune arm? Why do the two commonest errors — anchoring on the familiar rheumatic label, and treating a rheumatic-immune overlap with a biotic before the immune work-up — persist? [3] [4]