BACK TO IMMUNOLOGY BACK TO HOST INVASION BY MICROORGANISMS

Introduction

Although there are over 50,000 identified species of fungi, only a few hundred are known to be pathogenic, causing infections known as mycoses (mycology being the study of fungi). Fungi are nonphotosynthetic organisms with a eukaryotic cell structure and are able to grow as branching or filamentous forms (mycelia) or as single cells (yeasts).

Major pathogenic fungi

Superficial: These infections affect only the skin, hair, nails etc.

Generally not dangerous
Examples- Hortaea werneckii, causing tinea nigra, and Piedraia hortae, causing black piedra

Cutaneous: Also known as dermatophytoses

Only affect superficial keratinized tissue- skin, hair and nails
Again generally not dangerous
Examples- Candida albicans, causing candidiasis of the skin and nails, Microsporum species

Subcutaneous: Usually cause chronic localised infections of the skin and underlying dermis

Normally reside in soil and vegetation; they enter skin through cuts and grazes
Can occasionally affect the deep tissues, e.g. bones and muscles
Examples- Sporothrix schenkii, causing sporotrichosis, Madurella mycetomatis causing mycetoma

Systemic: Also known as deep, endemic or primary

Usually serious; sometimes fatal
Examples- Histoplasma capsulatum causing histoplasmosis, Blastomyces dermatitidis causing blastomycosis

Opportunistic: Ubiquitous fungi that affect only susceptible hosts; immunocompetent individuals are normally resistant

Examples- Aspergillus fumigatus, causing aspergilliosis, Cryptococcus neoformans, causing cryptococcosis

Immunity to fungi

Cutaneous and superficial fungal infections are normally self-limiting, with recovery associated with a resistance to re-infection. As hosts often develop delayed-type hypersensitivity, this resistance appears to be cell-mediated.

T cell immunity- it is thought that helper T cells release cytokines to activate macrophages to fight the infection
Neutrophils- there is now growing evidence for the role of neutrophils in some infections of the respiratory system, with defensin and the nitric oxide pathways implicated