Allergies are a consequence of the immune defense systems reacting to any antigen. A defense against this reaction is found in certain T cells:

https://www.sciencedaily.com/releases/2020/06/200612172234.htm

"Scientists offer a clue to why non-allergic people don't have a strong reaction to house dust mites. They've uncovered a previously unknown subset of T cells that may control allergic immune reactions and asthma from ever developing in response to house dust mites -- and other possible allergens.

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"In a new Science Immunology study, published on June 12, 2020, scientists at La Jolla Institute for Immunology (LJI) offer a clue to why non-allergic people don't have a strong reaction to house dust mites. They've uncovered a previously unknown subset of T cells that may control allergic immune reactions and asthma from ever developing in response to house dust mites -- and other possible allergens.

"Why house dust mites? These microscopic critters are hard to avoid, which means nearly everyone has been exposed. Even in people without a house dust mite (HDM) allergy, the immune system is likely to react in some way as it learns to recognize HDM molecules. This makes HDM a useful model for studying what causes allergies and asthma attacks.

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"Their analysis suggests that a subset of helper T cells, called interleukin (IL)-9 Th2 expressing HDM-reactive cells, is more prevalent in the blood of people with HDM-allergic asthma compared with those who are only allergic to HDM. Further analysis suggested that those IL9-TH2 cells are enriched in a group of molecules/genes that increased the cytotoxic potential of those cells. In other words, those specific T cells could kill other cells and drive inflammation.

"In contrast, another subset of T cells stood out in the non-allergic subjects. These T cells express an "interferon response signature" and were enriched for a gene that encodes a protein called TRAIL. The work done by Seumois and his colleagues suggest that TRAIL could be important because it could dampen the activation of helper T cells."

Comment: this is a protective mechanism that attempts to avoid allergic responses which are not necessary for immune defenses, but are mistakes. It must be part of an immune system design.