I’ve seen reports of this in the news a few times lately: some people can become allergic to red meat after being bitten by a tick. (See here or here, for example.) We are not used to thinking about allergic reactions this way. We think of sensitization to an allergen due to repeated exposure to that allergen. Peanut allergies come from exposure to peanuts, for example. So why does exposure to a tick bite sometimes lead to becoming allergic to red meat?
The connection is a carbohydrate called galactose-alpha-1,3-galactose, or just “alpha-gal”. Alpha-gal is present in the tissues of mammals, with the exception of primates (including humans). When a tick bites an animal, some alpha-gal then gets into the tick. When the tick bites a human, the alpha-gal is transferred through the tick’s saliva into the individual who is bitten. The bite, or perhaps a component of the tick’s saliva, triggers an immune response that results in the alpha-gal being recognized as a foreign substance leading to sensitization to alpha-gal upon later exposures. Since alpha-gal is present in animals, particularly those from which we get red meat, an individual who has become sensitized to alpha-gal has an allergic reaction to eating red meat.
In the United States, the problem seems to be closely associated with only the Lone Star Tick, which has a limited range mostly in the Southeast. The problem doesn’t appear to be currently widespread.
For details on how the cancer drug cetuximab was the key to discovering the connection between tick bites and red meat allergy, the paper titled “The relevance of tick bites to the production of IgE antibodies to the mammalian oligosaccharide galactose-α-1,3-galactose” makes a good read.