Alpha-Gal syndrome unveiled: decoding the glycan-driven red meat allergy

α-Gal syndrome (AGS) is a type of allergy to red meat and other mammalian products that develops after a tick bite. Symptoms may include urticaria, angioedema, gastrointestinal and respiratory symptoms, and, most dangerously, anaphylactic shock. Among food allergies, it is considered unusual because: (i) the major allergen is not a protein, but a carbohydrate moiety Galα-1,3-Gal, which humans do not synthesize; (ii) it is caused by α-Gal-containing glycoconjugates injected during a tick bite; (iii) the hypersensitivity is effected by IgE antibodies which arise after a class switch from IgG and IgM-class antibodies; (iv) it seems to be triggered by α-Gal-containing glycolipids present in red meat, with delayed onset. Here, we evaluate glycans that may cause AGS, review the data about glycosyltransferases that synthesize α-Gal and related antigens and describe the immune mechanism behind AGS: sensitization and host response to ticks' saliva. In addition, we consider future research avenues that may lead to a better understanding and management of AGS as a modern dietary affliction rooted in an old evolutionary trade-off.