Scientists have uncovered the genetic and molecular structure of essential aspects associated with the potentially fatal mammalian-meat allergy caused by tick bites.
The study, which was led by researchers at the Garvan Institute of Medical Research, shows how antibodies interact with the sugar molecule galactose-α-1,3-galactose (alpha-gal/-gal), which is made by all mammals except humans and higher primates. It further supports the idea that α-gal is the essential component in this particular allergy.
When people are exposed to α-gal through the bites of specific tick species, such as the Eastern Australian-endemic paralysis tick Ixodes holocyclus, the immune system may identify it as hostile and trigger an allergic reaction, which can occasionally have life-threatening effects.
The molecular research, according to the study’s lead author Professor Daniel Christ, Head of Antibody Therapeutics and Director of the Centre for Targeted Therapy at Garvan, revealed that a certain antibody type (3-7) has a natural pocket into which α-gal fits well.
“We have more than 70 types of antibodies,” says Professor Christ, adding “and this one is significantly overrepresented with α-gal recognition. We seem to be genetically predisposed to being sensitive to this sugar.”
The new study, which was just released in the PNAS journal (doi: 10.1073/pnas.2123212119), opens the door for possible therapy alternatives to treat the uncommon allergic reaction.
Immune response to α-gal: an evolutionary advantage
Researchers examined the blood of patients with mammalian-meat allergies to identify the antibodies that were produced; they discovered that the 3-7 type was usually formed in reaction to α-gal.
The evidence suggests that having an antibody response that can mobilize against α-gal has evolutionary advantages.
“Humans lost the capacity to produce α-gal throughout evolution, but we don’t know why,” adds Associate Professor Joanne Reed, co-senior author of this study, from the Westmead Institute.
“The suspicion is that it has to do with protection against infectious disease.”
Professor Christ cites new studies on malaria that demonstrate the Plasmodium parasite has a α-gal surface coating. The parasite could be eliminated quickly by the immune system in reaction to α-gal, preventing malaria.
New South Wales is a global epicenter for tick-induced mammalian-meat allergy
The northern part of Sydney is the most common place in the world to be allergic to mammalian meat, with more than 1800 reported cases. Another hotspot is the Queensland hinterland along the Sunshine Coast near Maleny. These regions are home to the Ixodes holocyclus paralysis tick.
The first clinician to connect tick bites with mammalian-meat allergy was Professor Sheryl van Nunen, an allergy specialist at Sydney’s Northern Beaches Hospital and a co-author of the study.
She adds: “There isn’t a week go by that I wouldn’t see two people with this allergy.”
No one knows why some people get anaphylaxis and others don’t. According to Professor van Nunen, it may be related to the number of tick bites, the amount of saliva injected, or hereditary susceptibility.
According to Professor van Nunen, exposure occurs when α-gal, a substance found in the saliva of some tick species, is injected during a bite.
She says that about one-third of individuals who have developed a sensitivity to α-gal will also show signs of an allergy to mammalian meat.
Furthermore, a second bite can more than double the adverse reaction. The inclusion of meat components in food, such as beef broth, soft cheeses like feta or goat’s cheese, or even gelatin, might have an adverse effect on some persons with severe allergies.
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