Recently, it’s been suggested that autoimmune disorders are the result of our bodies overreacting as an adaptation to parasites that produced immunosuppressants. Achieve a high enough parasitic load and you’re effectively immunocompromised and soon dead, unless your immune system started out turned up to 11 and dipped down to a natural level with a heavy dose of immunosuppressants via contemporaneously common parasites. That’s the gist, here’s the wiki link.

https://en.wikipedia.org/wiki/Helminthic_therapy

Adding on to the other comment, it's one of the theories for why allergies exist

Each of our antibodies seem to have a specific job, and that allows them to call for the correct response for the type of threat they are dealing with (ex. Fighting one large thing vs many small things, where in the body is it happening, what kind of cells are involved, etc)

Of those, IgE is the one that's tied to allergies. Turns out it's also involved in the response against parasitic worms. Parisitic worm infections aren't as common in many parts of the modern world, while allergies are. So maybe we can treat allergies by studying worm infections

The classes differ in their biological properties, functional locations and ability to deal with different antigens, as depicted in the table.[19] For example, IgE antibodies are responsible for an allergic response consisting of histamine release from mast cells, often a sole contributor to asthma (though other pathways exist as do symptoms very similar to yet not technically asthma). The variable region of these antibodies bind to allergic antigen, for example house dust mite particles, while its Fc region (in the ε heavy chains) binds to Fc receptor ε on a mast cell, triggering its degranulation: the release of molecules stored in its granules.[46]

https://en.wikipedia.org/wiki/Antibody

https://en.wikipedia.org/wiki/Immunoglobulin_E