Immunocompromise and Susceptibility to Fungal Infection

Immunocompromise is the state of impaired host defence that converts ordinarily contained or commensal fungi into agents of serious, often invasive disease. Because most fungi are opportunists, susceptibility to fungal infection is determined largely by the integrity of the host's immune system, and different immune defects map predictably to different fungal infections.

Definition

Immunocompromise denotes any congenital or acquired impairment of immune function that diminishes antifungal defence and thereby increases susceptibility to opportunistic and invasive fungal infection.

Scope

This topic covers the principal forms of immunocompromise that raise fungal risk—neutropenia and neutrophil dysfunction, T-cell deficiency, inborn errors of immunity affecting antifungal pathways, and iatrogenic immunosuppression—and the concept that specific defects predispose to specific fungal infections. It is a reference and educational entry on host susceptibility, not clinical guidance on prophylaxis or treatment.

Core questions

• Why are fungal infections predominantly diseases of the immunocompromised?
• How do different immune defects predispose to different fungi?
• What is the difference between opportunistic and primary fungal pathogens?
• How large is the global burden of fungal disease, and who bears it?

Key concepts

• Opportunistic fungal infection
• Neutropenia and phagocyte defects
• T-cell (CD4+) deficiency
• Inborn errors of immunity
• Iatrogenic immunosuppression
• Defect-to-infection mapping
• Invasive versus mucosal fungal disease

Mechanisms

Antifungal protection depends on intact innate and adaptive defences, so their failure opens distinct windows of susceptibility. Loss of neutrophils or of phagocyte oxidative killing predisposes especially to invasive mould and yeast infections, while impairment of CD4+ T-cell and Th17/IL-17 responses predisposes to mucosal and chronic mucocutaneous fungal disease. Human cohorts of patients with defined inborn errors of immunity have shown that particular molecular defects map to particular fungal infections, revealing the non-redundant roles of individual antifungal pathways (lionakis-2018; netea-2015). Iatrogenic immunosuppression—from transplantation, chemotherapy, corticosteroids, and biologic therapies—reproduces these vulnerabilities and accounts for much modern invasive fungal disease (brown-2012).

Clinical relevance

Recognising that fungal susceptibility tracks specific immune defects clarifies why invasive mycoses concentrate in patients with neutropenia, transplantation, advanced HIV, or particular genetic immunodeficiencies. This topic explains those host-risk relationships for educational orientation and does not provide thresholds, prophylaxis regimens, or treatment recommendations for individual patients.

Epidemiology

Serious fungal infections affect a large global population, with millions of cases of invasive and chronic mycoses estimated each year, the burden falling disproportionately on immunocompromised hosts; multinational prevalence estimates have been compiled to quantify this under-recognised burden (bongomin-2017; brown-2012).

Evidence & guidelines

The account synthesises narrative reviews, human cohort findings, and population-level prevalence estimates (lionakis-2018; brown-2012; bongomin-2017). Management of fungal infection in immunocompromised patients is governed by separate infectious-disease guidelines outside the scope of this entry.

History

The link between immune deficiency and fungal disease became prominent with the rise of intensive chemotherapy, organ and stem-cell transplantation, and the HIV/AIDS epidemic, all of which expanded the population at risk for invasive mycoses. Later, the study of inborn errors of immunity refined the picture by showing that discrete genetic defects predispose to discrete fungal infections, and global burden surveys quantified the scale of the problem (brown-2012; lionakis-2018; bongomin-2017).

Key figures

• Michail S. Lionakis
• Stuart M. Levitz
• Gordon D. Brown
• David W. Denning
• Mihai G. Netea

Seminal works

• lionakis-2018
• brown-2012
• bongomin-2017

Frequently asked questions

Why do fungal infections mainly affect immunocompromised people?

Most fungi are opportunists that intact immune defences normally contain; when defences such as neutrophils or T-cell responses fail, these fungi can invade and cause serious disease.

Do different immune defects cause different fungal infections?

Yes. Neutrophil defects predispose particularly to invasive mould and yeast infections, whereas T-cell and IL-17 pathway defects predispose more to mucosal and chronic mucocutaneous fungal disease.