Reading the Infection Pattern | AI Learning Guide

Use the three columns as a decision guide. Start with the organism. Encapsulated bacteria in blood or meninges point to complement or antibody failure. Catalase-positive organisms in abscesses point to a phagocyte killing defect. Gram-negative organisms colonizing a damaged surface point to a barrier or clearance problem. Then check the site and the age of onset. Complement deficiency often presents in infancy with invasive disease, while isolated antibody deficiency appears later when maternal antibodies fade. Recurrent Neisseria infection is a specific clue for a terminal complement component. The pattern does not give you a diagnosis by itself, but it tells you which layer to investigate first, so you order the right test instead of testing everything.

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Defect to infection pattern

Barrier / clearance defect

Organisms: Pseudomonas, Staphylococcus aureus, Haemophilus influenzae
Site: airway, sinuses, ears, skin wounds, urinary tract
Pattern: chronic or recurrent local infection at the affected surface
Adaptive immunity intact, so bloodstream infection is handled normally

Complement deficiency

Organisms: encapsulated bacteria (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis)
Site: blood, meninges, joints
Pattern: invasive infection, often in infancy for C3 deficiency
Terminal component (C5-C9) deficiency: selective Neisseria vulnerability

Phagocyte killing defect

Organisms: catalase-positive (Staphylococcus aureus, Serratia, Burkholderia, Aspergillus)
Site: skin, lymph nodes, liver, lungs
Pattern: recurrent abscesses and granulomas from early childhood
Ingestion normal, intracellular killing fails

Why the pattern is predictable

Each layer handles a specific gap. Barriers and clearance keep organisms out of tissue. Complement opsonizes and lyses organisms that reach the blood. Phagocytes kill what they ingest. When one layer fails, the organisms that depend on that layer for control are the ones that cause disease. This is why the infection pattern is not random: it reflects which defense is missing.

Overlap is possible

A single patient can have defects in more than one layer, and some organisms can be controlled by multiple mechanisms. The pattern is a guide to the most likely layer, not a definitive test.