system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we get allergies? Is there an evolutionary reason/benefit for why they exist?

So it seems that with most things with animals, there's some form of reason for why each thing exists or happens. So if we take hayfever for example, is there any reason why this may have come to be so common?
gpt
While it may seem that way, there's actually many examples of traits and behaviours (especially at the individual level) that just *are*. With ~24,000 protein coding genes, approximately double that number of non-coding RNAs and exponentially more regulatory regions - random, disadvantageous mutations will happen. As long as the mutation or other circumstance doesn't prevent the carrier from passing it on (lethality, infertility), it can stay in the population through multiple generations at low or modest frequency. You don't need to be the most fit, or most successful to survive. Many mutations are balanced as well, in that they may be good in one context, but not great in a different one.

Allergies are caused by a hypersensitivity of the immune system. 

Your immune system function is controlled by many, many genes, and also an element of both randomness and prior exposure. Immune cells are "trained" in the thymus to avoid self-reactions (autoimmunity), or "trained" by prior exposure (including vaccines, environmental exposure) to react to certain other pathogen antigens. Sensitivity of the immune system is absolutely a good thing, allowing protection against a huge variety of pathogens, including completely new ones that only need to be recognized as "non-self".

Allergic reactions are specifically caused by hypersensitivity of mast cells and basophils which - in allergies - react strongly and specifically to foreign proteins that are actually benign. The cells and pathways being activated however, are exactly the same as those you want activated if you're exposed to a real threat. If you were to dampen these across the board, you risk becoming more susceptible to real infectious threats which have been a huge evolutionary pressure throughout history.

Balancing your immune system between too much and too little inflammation, as well as too much and too little reactivity is at the root of many conditions and disorders, as well as being responsible for differential responses to infection (why some people die when exposed to influenza, tuberculosis, malaria, plague, ebola, etc while others survive and recover). Humanity exists along a continuum. Those with allergies are just tipped a little too high in a particular direction.