Antibody deficiencies — branching viva

Opening question

Take me through your framework when a child presents with recurrent infection that you suspect might be due to an antibody deficiency. [1] [2]

Branch 1 — the likely diagnosis

The boy has absent tonsils and lymph nodes, pan-hypogammaglobulinaemia, and absent B cells, presenting after maternal antibody wanes. What is the most likely diagnosis, which gene is implicated, and why does the timing fit? [3]

Branch 2 — confirming the defect

How will you confirm this diagnosis, and why is a functional vaccine response important even though the immunoglobulins are already clearly low? What does the absence of B cells tell you that the low IgG alone does not? [2] [6]

Branch 3 — long-term management

Outline the principles of long-term management: immunoglobulin replacement, infection prophylaxis, lung surveillance, and genetic counselling for the mother's carrier relatives. What is the prognosis with consistent therapy? [3] [4]

Branch 4 — the contrasting case (the trap)

Now picture a thriving two-year-old with a single low IgG but a preserved vaccine response and B cells present. Why would you NOT start immunoglobulin here, and what is the expected natural history? What would make you escalate? [6] [4]

Closing — the general rule

In one sentence, what is the single rule that decides whether a child with low immunoglobulins receives immunoglobulin replacement, and why does over-treatment matter as much as under-treatment? [2] [6]