The upside to allergies: Cancer prevention

A new article in the December issue of The Quarterly Review of Biology provides strong evidence that allergies are much more than just an annoying immune malfunction. They may protect against certain types of cancer.

The article, by researchers Paul Sherman, Erica Holland and Janet Shellman Sherman from Cornell University, suggests that allergy symptoms may protect against cancer by expelling foreign particles, some of which may be carcinogenic or carry absorbed carcinogens, from the organs most likely to come in with contact them. In addition, allergies may serve as early warning devices that let people know when there are substances in the air that should be avoided.

Medical researchers have long suspected an association between allergies and cancer, but extensive study on the subject has yielded mixed, and often contradictory, results. Many studies have found inverse associations between the two, meaning cancer patients tended to have fewer allergies in their medical history. Other studies have found positive associations, and still others found no association at all.

In an attempt to explain these contradictions, the Cornell team reexamined nearly 650 previous studies from the past five decades. They found that inverse allergy-cancer associations are far more common with cancers of organ systems that come in direct contact with matter from the external environment - the mouth and throat, colon and rectum, skin, cervix, pancreas and glial brain cells. Likewise, only allergies associated with tissues that are directly exposed to environmental assaults - eczema, hives, hay fever and animal and food allergies - had inverse relationships to cancers.

Such inverse associations were found to be far less likely for cancers of more isolated tissues like the breast, meningeal brain cells and prostate, as well as for myeloma, non-Hodgkins lymphoma and myelocytic leukaemia.

The relationship between asthma and lung cancer, however, is a special case. A majority of the studies that the Cornell team examined found that asthma correlates to higher rates of lung cancer. 'Essentially, asthma obstructs clearance of pulmonary mucous, blocking any potentially prophylactic benefit of allergic expulsion,' they explain. By contrast, allergies that affect the lungs other than asthma seem to retain the protective effect.

So if allergies are part of the body's defence against foreign particle invaders, is it wise to turn them off with antihistamines and other suppressants? The Cornell team says that studies specifically designed to answer this question are needed.

'We hope that our analyses and arguments will encourage such cost/benefit analyses,' they write. 'More importantly, we hope that our work will stimulate reconsideration... of the current prevailing view... that allergies are merely disorders of the immune system which, therefore, can be suppressed with impunity.'