Alpha-gal is a carbohydrate that most mammals make but humans, apes and Old World monkeys do not, having lost the enzyme. Tick saliva contains alpha-gal, and in some people repeated exposure through bites provokes an immunoglobulin E response against it. Once sensitised, eating mammalian meat can trigger hives, gastrointestinal symptoms or anaphylaxis.
The timing is the diagnostic oddity. Most food allergies react within minutes; alpha-gal syndrome typically produces symptoms two to six hours after eating, because the allergen is carried in dietary fats that take that long to reach the circulation. The delay meant the link went unrecognised for years, with reactions blamed on anything but dinner.
Alpha-gal syndrome was first characterised in the late 2000s and has since been reported in association with several tick species on multiple continents. Why only some bitten people become sensitised, whether sensitivity fades if further bites are avoided, and the full geographic range of the syndrome are all still being worked out.