We can't conclude that the donor had an egg allergy. The immune system is complicated as hell, it could be the case that he had some weird protein circulating on his blood that was recognized by her immune system, and by chance this protein was structurally close enough to some egg protein that now eggs trigger an immune response on her.

Worth noting that the most prevalent protein in blood is albumin, and the major protein in eggs is, yep, albumin.

Are you implying there is a connection? Even if it's just for nonsense, I'd love to read it. Because other prevalent commonalities, you could make the same argument about water, for example.

"The immune system is complicated as hell."

You got that right! My wife used to eat a banana every day. Then something in her flipped and she reacts as if she is allergic to bananas and the rest of the world - not exaggerating. She can eat a very small number of foods and has to avoid most people because of smells. It isn't allergies it is - you guessed it - her immune system.

If we could just flip it back to "normal"...

Same thing happened to me. I was eating bananas, beef, coffee, etc. then I got some sort of viral infection. My thyroid became inflamed for a couple months. No doctor was able to find anything wrong with me.

Little did I know, having postviral sequelae causes my immune system to start hating many things I used to eat. A food blood sensitive test showed that bananas, beef, coffee were what set my immune system off the most.

I was told it was gut permeability causing food particles to leak into the blood stream causing the immune response. So the fix has been to stop eating those, let the gut heal, and over time I've been able to eat those foods in moderation years later.

Did the smell of a banana, or just having one in the house set you off - or did you have to eat it to have a response?

Only eating bananas

I don't think I ever had to declare allergies when donating my blood (France).

They focus on infections and cancers I think.

Why would they ask for allergies if we don't yet know that they can be transmitted like this?

I thought it was well known that certain allergies (eggs/peanuts) can transfer via transfusions? At least for awhile. It looks like there’s some literature on it at least, but I didn’t dig into it or anything. I just thought this was “common” knowledge since that’s what I was told years ago by a nurse I was dating in college, a long time ago.

I would assume things like that maybe after a bone marrow transplant but having a blood transfusion seems pretty mild I wouldn't expect any affects from it.

Maybe it's known but I don't know about it :-)

Some 30% of people have allergies. It can already be challenging to source enough blood with the current eligible population.