What Is Soy Lecithin And Is It Harmful To Someone Who Is Allergic To Eggs?

It's probably okay for the egg-allergic person. Personally I would still consult with an allergist to be sure, depending how life-threatening the allergy can be.  You cannot and should not rely on the medical-related advice you get on Blurtit.

Lecithin is basically a naturally produced emulsifier or surfacent. Chemically it's a fat, and it's function is to bind certain types of stuff together (like fats and water).  Lecithin is naturally produced by human bodies, many animals and some plants.

If a product says it contains "soy lecithin" then the lecithin comes from soy products (soya is a fairly undigestible bean), and should not be expected to irritate people with egg allergies.  However, in some cases, lecithin is derived from egg yolk, so that would be unsuitable.

Basically, if you see "lecithin" on the ingredient list, assume it's egg-derived, unless it stays something like "soy lecithin" or "lecithin from plant sources", some wording that suggests it's not from eggs.