At this time, there is insufficient data to prove this. Onion has, indeed some benefic health effects:
Compounds from onion have been reported to have a range of health benefits which include anticarcinogenic properties, antiplatelet activity, antithrombotic activity, antiasthmatic and antibiotic effects .
This is supported by another study:
From these results, we conclude that NPTS (n-propyl thiosulfate) and 2PTS (2-propenyl thiosulfate) have the potential to promote immune functions and prevent cardiovascular diseases .
The immune response was tested on rabits, and it was concluded that onion has antiinflammatory properties:
It was found that mean antibody titers were significantly lower in the treated groups than in controls. The weights of thymus and lymph nodes were higher and of adrenal glands were lower in the control groups than in the treated groups. It appeared from the current study that onion extract has an inhibitory effect on immune response .
There is even a study on onion poisoning conducted on dogs. It claims onion can induce hemolytic anemia .
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- Chang HS, Yamato O, Sakai Y, Yamasaki M, Maede Y. Acceleration of superoxide generation in polymorphonuclear leukocytes and inhibition of platelet aggregation by alk(en)yl thiosulfates derived from onion and garlic in dogs and humans. Prostaglandins Leukot. Essent. Fatty Acids. 2004 Jan;70(1):77-83. PubMed PMID: 14643182.
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- Tang X, Xia Z, Yu J. An experimental study of hemolysis induced by onion (Allium cepa) poisoning in dogs. J. Vet. Pharmacol. Ther. 2008 Apr;31(2):143-9. doi: 10.1111/j.1365-2885.2007.00930.x. PubMed PMID: 18307506.