Can one with a soy allergy be treated with soy lecithin to build an immunity?

No : Lecithin is fat and contains no soy protein. Generally people allergic to soy protein can eat soy lecithin without fear on an allergic reaction. There is no evidence that eating soy lecithin creates tolerance to soy protein. Sorry. Great question.

Probably not: We generally do not instruct patients with soy allergy to avoid soy lecithin. Processed soybean oil is considered safe in patients with soy allergy. No cure for food allergy is available. Strict avoidance of the food allergen is the only therapeutic option. There are some experimental strategies on the horizon, though, such as oral, sublingual, and subcutaneous immunotherapy.