Researchers at New York's Cornell University have found allergies can help in the fight against cancer by expelling carcinogenic particles from the body.

Allergies may also serve as an early warning to let people know when substances in the air could be dangerous and should be avoided.

Researchers have previously suspected an association between allergies and cancer, but past results have been inconclusive.

A number of studies found some cancer patients have fewer allergies in their medical history, while other research projects found the reverse to be true.

The Cornell researchers re-examined 650 studies undertaken over the past five decades and found allergy-cancer associations more common with organ cancer systems that come into direct contact with matter from the external environment.

This included the skin, cervix, mouth, pancreas, colon and glial brain cells. Allergies associated with tissues exposed to the environment such as eczema, hay fever and food allergies were found to protect against certain cancers.

But these allergies were found to be less effective against cancers of more isolated tissues such as the breast and prostate, myeloma, non-Hodgkins lymphoma and myelocytic leukemia.

The relationship between asthma and lung cancer bucked the trend.

The Cornell team discovered the majority of studies found asthma correlated to higher rates of lung cancer.

''Essentially, asthma obstructs clearance of pulmonary mucous, blocking any potentially prophylactic benefit of allergic expulsion,'' the article said.

Allergies that affected the lungs other than asthma retained the protective effect or decreased risk.

The researchers advised those who were allergy-free not to fear that they would develop cancer, as there were many reasons why some people didn't have allergy symptoms.

They also called for more study into the rampant use of antihistamines and other allergy suppressants.

''We hope that our analyses and arguments will encourage such cost-benefit analyses,'' the article said.

''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.''

The researcher's article will appear in the December issue of The Quarterly Review of Biology.