I’m going to say it’s X linked agammaglobulinemia rather than SCID, but the difference between these two are tiny but this is why I think it’s the former:

• Boy (increased risk but both BA and SCID are x linked)
• Recurrent bacterial infections but don’t mention diarrhea or thrush which is in SCID
• Timeline is after 6 months, so the mother’s antibodies wore off.

SCID should be immediately because they just don’t have the IL2 receptors. CVID shows up when they’re 20-40 years old. You get absent germinal centers in both. No mention of absent thymic shadow which is in SCID.

I think I found what the disease was, though I honestly have no idea why they would test this rather than XLA. There’s a condition called Transient Hypogammaglobulinemia of Infancy. It presents w/low immunoglobulin levels post 6 months and can present w/small lymph nodes and tonsils in infancy BUT w/o any other findings of primary immunodeficiency including decreased counts.

Here’s an article about it: https://primaryimmune.org/about-primary-immunodeficiencies/specific-disease-types/transient-hypogammaglobulinemia-of-infancy/

You definitely don’t need to know the disease to get the correct answer since the link of lack of immunoglobulins would clue you into the lack of germinal centers, but I think this is more likely than XLA since every source I read implies that B cell counts are near 0 in the classic presentation (unless I’m missing a reason why leukocyte count w/diff wouldn’t show a significant decrease in lymphocytes due to near-zero B cells). Just wanted to put this here in case other people later came wondering, though I may still be wrong.

Male child, recurrent infections, no mature B cells = Bruton disease (X-linked agammaglobulinemia)

Common variable immunodeficiency - Defect in B-cell differentiation. Cause unknown in most cases. May present in childhood but usually diagnosed after puberty. Increased risk of autoimmune disease, bronchiectasis, lymphoma, sinopulmonary infections Decreased plasma cells, Decreased immunoglobulins