The vast majority of fungi are free-living, but a few can infect larger animals, colonizing the skin or entering via the lung in the form of spores (centre left). Fungal infections are normally only a superficial nuisance (e.g. ringworm, top), but a few fungi can cause serious systemic disease, particularly if exposure is intense (e.g. farmers) or the immune system is in some way compromised (e.g. AIDS); the outcome depends on the degree and type of immune response, and may range from an unnoticed respiratory episode to rapid fatal dissemination or a violent hypersensitivity reaction.
In general, the survival mechanisms of successful fungi are similar to those of bacteria: antiphagocytic capsules (e.g. Cryptococcus), resistance to digestion within macrophages (e.g. Histoplasma) and destruction of polymorphs (e.g. Coccidioides). Some yeasts activate complement via the alternative pathway, but it is not known if this has any effect on survival.
Perhaps the most interesting fungus from the immunological point of view is Candida albicans (upper left), a common and harmless inhabitant of skin and mucous membranes which readily takes advantage of any weakening of host resistance. This is most strikingly seen when polymorphs (PMN) or T cells are defective, but it also occurs in patients who are undernourished, immunosuppressed, iron deficient, alcoholic, diabetic, aged or simply ‘run down’ (see Fig. 33). Organisms that thrive only in the presence of immunodeficiency are called ‘opportunists’ and they include not only fungi, but also several viruses (e.g. CMV), bacteria (e.g. Pseudomonas), protozoa (e.g. Toxoplasma) and worms (e.g. Strongyloides), and their existence testifies to the unobtrusive efficiency of the normal immune system.
The most important ectoparasites (‘outside living’; skin dwelling) are mites, ticks, lice and fleas. The last three are vectors for several major viral and bacterial diseases. The evidence for immunity, and the feasibility of a vaccine, are currently under intense study.
PMN
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