Primary and secondary immune deficiencies in children

1. Big picture

Immune deficiency in pediatrics means the child’s immune system cannot protect normally against infection. The exam question usually gives you a child with recurrent, severe, unusual, persistent, or treatment-resistant infections.

Core clinical pattern:

Too many infections + severe infections + unusual organisms + poor growth
→ think immunodeficiency

The key exam skill is to identify which immune arm is defective:

Immune arm	Main infection pattern
B-cell / antibody defect	Recurrent sinopulmonary bacterial infections, especially encapsulated bacteria
T-cell defect	Viral, fungal, opportunistic infections; chronic diarrhea; failure to thrive
Combined B + T defect	Severe infections from early infancy; life-threatening
Phagocyte defect	Deep abscesses, lymphadenitis, catalase-positive organisms
Complement defect	Neisseria infections, autoimmune disease
Secondary immunodeficiency	Immunodeficiency due to another disease, drug, malnutrition, HIV, malignancy, etc.

Core oral-exam sentence: “In children, immunodeficiency should be suspected when infections are recurrent, severe, unusual, persistent, complicated, or associated with poor growth or family history.”

1. Big picture

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