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I just read an NPR article about allergies to red meat being caused by tick bites. That stood out to me because I thought allergies are triggered by an initial exposure to some allergen. In this case, the allergen is alpha-gal, a disaccharide found in cell membranes of most mammals; humans and apes are a notable exception. The article explains that ticks might cause this allergy by transferring alpha-gal from other animals to us when they feed on our blood.

However, we also are exposed to alpha-gal every time we eat red meat. How are we protected from allergic reactions when we eat red meat but not when we eat peanuts? Why does that protection fail after alpha-gal is exposed to our bloodstream?

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    $\begingroup$ Please give the full chemical name for the compound you refer to as alpha-gal. This is not a common abbreviation. $\endgroup$ – David Jul 25 '18 at 4:18
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    $\begingroup$ @David Gal is the standard abbreviation for galactose, per IUPAC. $\endgroup$ – canadianer Jul 25 '18 at 5:00
  • $\begingroup$ @canadianer — The question referred to alpha-gal which is not, but in any case glc is the IUPAC abbreviation for glucose but it would be strange to use this. My point is that for optimal communication (and especially in titles) abbreviations should only be used if they are widely known (that is what I had intended) and really necessary. $\endgroup$ – David Jul 25 '18 at 13:23
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    $\begingroup$ @David the allergy itself is known by clinicians as alpha-gal syndrome. A common (but unclear) name would be meat allergy. I think the title is clear to people that are familiar with it. $\endgroup$ – De Novo Jul 25 '18 at 16:08
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In order for the immune system to respond to an antigen, beyond recognition of that antigen, there needs to be a signal that there is something wrong. Co-stimulation can take a few different forms, but it requires the involvement of more than just one arm of the immune system, and generally helps prevent our immune system from what I like to call the Dr. Strangelove effect (a psychotic lone general can just decide that there is a problem and launch an attack). Here, costimulation might be from (or downstream of) the tick bite and other invaders associated with the tick bite, or from an allergy to peanuts. You can read about this in the first chapter of Sompayrac's How the Immune System Works, as well as the first chapter of Abbas Basic Immunology.

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  • $\begingroup$ Does the immune response also vary by the route that the antigen enters the body? I'm speculating here, but if the GI tract is less prone to triggering immune responses, that could explain why we're okay with eating alpha-gal but not if alpha-gal gets in our blood. While it's possible that the tick bite itself is a source of some costimulating substance, I also read that immune responses against alpha-gal may also play a role in rejection of pig tissue implants. I can't imagine that a similar response would occur if we ATE that tissue. $\endgroup$ – BatWannaBe Jul 26 '18 at 22:38
  • $\begingroup$ @BatWannaBe yes, mucosal exposure is less immunogenic, and eating pork is different from having a porcine valve. Costimulation is a requirement, though, however the antigen is introduced. $\endgroup$ – De Novo Jul 26 '18 at 23:36

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