Host Defense and Infections in the Immunocompromised Host
Core concepts
Host defense defects predict pathogens. Neutropenia: gram-negative rods, S. aureus, Aspergillus. Asplenia: encapsulated bacteria (pneumococcus, meningococcus, H. influenzae type b). Complement C5-C9 deficiency: recurrent Neisseria. Antibody deficiency: sinopulmonary infections with encapsulated organisms; treat with immunoglobulin. Phagocyte defects: chronic granulomatous disease (catalase-positive organisms such as S. aureus, Serratia, Aspergillus, Nocardia), leukocyte adhesion deficiency (no pus, delayed cord separation). T-cell defects and SCID: viruses, Pneumocystis, fungi, and live-vaccine disease. Fever with neutropenia is an emergency: obtain cultures and give an empiric anti-pseudomonal beta-lactam within 60 minutes.
Traps
- Corticosteroids can mask fever and signs of infection.
- Do not delay antibiotics to obtain cultures in febrile neutropenia.
- A normal white blood cell count does not exclude neutropenia; check the absolute neutrophil count.
- Live vaccines are contraindicated in SCID and severe cellular immunodeficiency.
Worked example
A 7-year-old with chronic granulomatous disease presents with fever and a lung nodule. Cover for Aspergillus and Nocardia (for example, voriconazole plus trimethoprim-sulfamethoxazole) and pursue biopsy and cultures.
Rule of thumb: identify the immune defect, predict the organism, and treat early; then prevent recurrence with prophylaxis and vaccines.