Secondary immunodeficiency — branching viva

Opening question

Take me through your framework when a child presents with infection that you suspect is due to secondary immunodeficiency. Why does the mechanism matter more than the label "immunocompromised"? [1] [2]

Branch 1 — the immediate threat (febrile neutropenia)

The child is febrile and profoundly neutropenic on chemotherapy. What is the immediate threat, what empirical management is required within the hour, and why is this treated as sepsis rather than observed? [1] [4]

Branch 2 — predict the organism (the steroid case)

Now picture a child on high-dose corticosteroids for four weeks presenting with dry cough and hypoxia. Which organism do you predict, which prophylaxis should already have been in place, and at what dose-duration threshold? [4] [1]

Branch 3 — the trap (rituximab and low IgG)

A child treated with rituximab for nephrotic syndrome now has a low IgG. Why would you NOT automatically start immunoglobulin, what decides whether replacement is indicated, and what is the expected natural history of the B-cell depletion? [3] [2]

Closing — the general rule

In one sentence, what is the single rule that decides whether a child with secondary hypogammaglobulinaemia receives immunoglobulin replacement, and why does over-treatment matter as much as under-treatment? [3] [1]