allergoid

noun

al·​ler·​goid ˈa-lər-ˌgȯid 

: an allergen (such as pollen) that has been chemically modified and is used especially in immunotherapy to treat allergic conditions by lessening the immune response

An allergoid made from ragweed allergen and injected into someone with hay fever could teach ragweed-specific [T helper] cells not to give the "go ahead" signal for a reaction to real ragweed pollen.

—Johnathan Brostoff and Linda Gamlin, Hay Fever: The Complete Guide …

Note: Allergoids are less likely to bind to the immunoglobulin IgE and cause serious side effects during treatment.