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At lunch one day, my friend had a hard boiled Easter egg. When he pealed it he squeezed the yolk, or yellow part, of the egg out. I asked him what on earth he was doing and he said, "I'm allergic to the yellow part of the egg not the white." I was very confused by this, he also sad he was allergic to tomatoes but could still eat pizza with sauce on it, apples but only the skins, and some other strange things. I didn't think this was possible at all. Can anyone explain this and tell me if it's even possible to be allergic to one part of an egg and not the other, or is it all in his head. If this is possible, is there an actual name for this condition?

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    $\begingroup$ It's plausible, on first thought, if the yolk contains different antigens than the white or the antigen in tomatoes is heat labile. $\endgroup$ – canadianer Apr 11 '18 at 20:25
  • $\begingroup$ This isn't really a biology point but I've known a few people who use "allergic" as a shorthand for any similar condition (e.g. lactose intolerance) so your friend may not necessarily have an allergy (in the sense of an antibody mediated reaction) $\endgroup$ – Jam Apr 11 '18 at 20:53
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The protein composition of the egg white and egg yolk differ appreciably. Hence, it is plausible for one to countenance symptoms of indigestion when consuming the egg yolk as opposed to the albumen, egg whites. It is important to stress that your friend may only an indigestion towards egg yolks as opposed to an immune response to an antigen.

If we consider the protein family, the egg yolk comprises a family of phosvitins, a group of highly phosphorylated proteins capable of mustering iron and calcium metal cations - serving a role in embryo development.

These are primarily absent from the albumen which comprises (with an unexpected name!) a highest percentage of suspended ovalbumin, ovotransferrin and ovomucoid. As demonstrated by the American College of Allergy, Asthma & Immunology, it is feasible to suffer an intolerance towards the egg yolk and yet suffer no gastro-enteric indigestion to the egg whites. More information regarding the American College is available from: https://acaai.org/allergies/types/food-allergies/types-food-allergy/egg-allergy

Your friend evidently does not suffer a severe anaphylactic shock after consumption of the egg yolks otherwise he/she may be adverse to both the egg whites and egg yolk. In conclusion, it is possible that he/she may simply have a mild intolerance towards certain proteins in the egg yolks.

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