B cell, T cell, combined, phagocyte or complement
Mnemonic
Match the infection to the arm of the immune system that has failed:
- B cell (antibody) - recurrent bacterial infections, especially encapsulated organisms and sinopulmonary infection, starting after 6 months as maternal IgG wanes. Bruton’s X-linked agammaglobulinaemia, common variable immunodeficiency, selective IgA deficiency (the commonest of all)
- T cell - viral, fungal and opportunistic infections, failure to thrive from birth. DiGeorge syndrome (22q11: thymic aplasia, hypocalcaemia, cardiac defects)
- Combined - SCID, presenting in infancy, requiring bone marrow transplantation
- Phagocyte - abscesses and catalase positive organisms. Chronic granulomatous disease, diagnosed by the nitroblue tetrazolium or dihydrorhodamine test
- Complement - early components: immune complex disease and lupus. Terminal components C5 to C9: recurrent Neisseria infections
“Bacteria means B cells, viruses and fungi mean T cells, abscesses mean phagocytes, Neisseria means complement.”
Live vaccines are contraindicated in T cell and combined deficiencies.
Expansion
| Arm | Example | Typical infections |
|---|---|---|
| B cell (antibody) | Bruton X-linked agammaglobulinaemia; selective IgA deficiency (commonest); common variable immunodeficiency | Encapsulated bacteria, recurrent sinopulmonary infection, giardia |
| T cell | DiGeorge (22q11 deletion, absent thymus) | Viruses, fungi, Pneumocystis, mycobacteria |
| Combined | SCID | Everything; presents in infancy, fatal without transplant |
| Phagocyte | Chronic granulomatous disease (NADPH oxidase), leucocyte adhesion deficiency | Catalase-positive organisms: Staphylococcus, Aspergillus, Serratia |
| Complement | C5 to C9 deficiency; C1 esterase inhibitor deficiency | Neisseria; hereditary angioedema |
Antibody deficiencies typically present after 6 months, once maternal IgG has waned, whereas severe T cell defects present earlier.
Warning signs prompting investigation: recurrent severe infections, unusual organisms, failure to thrive, family history, and infection with live vaccines.
Two specific associations worth remembering: DiGeorge with hypocalcaemia and cardiac outflow defects, and C1 esterase inhibitor deficiency causing angioedema that does not respond to adrenaline, steroids or antihistamines.