Wikipedia[1] has footnotes for this quote: “If a tick feeds on another mammal, the alpha-gal remains in its alimentary tract. The tick then injects the alpha-gal into a person's skin, which causes the immune system to release a flood of IgE antibodies to fight the foreign carbohydrate. Researchers still do not know which specific component of tick saliva causes the reaction.”

1 - https://en.wikipedia.org/wiki/Alpha-gal_allergy?wprov=sfti1