system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do people become allergic to things when they use to not be?

For example, I use to not be allergic to cats until I was about 17 years old, then one day my eyes started puffing up after about 15 minutes of being around them. Since then I have become allergic to multiple things.
gpt
The medical and science communities are still not sure exactly *why*, or what causes our bodies to flag something as an invader that it previously hasn't. When we develop an allergy can be affected by many variables, including genetics. If you have an allergy prone parent, you're 40% more likely to develop allergies. If both of your parents have allergies, you're 70% more likely. However, you may not have the same allergies they do. 

Allergies are a response of the immune system to what it sees as an invader. The circumstances have to be just right for the allergen to trigger an allergic reaction, so the first time you're exposed to cats you may be fine. The 17th time, you may be fine. The 70th time, you may be fine...but then the 71st time you have a reaction. It begins with exposure. Even if you've inhaled an allergen many times before with no trouble, at some point, for some reason, the body flags it as an invader. During this particular exposure, the immune system studies the allergen. It readies itself for the next exposure by developing antibodies, special cells designed to detect it. You are now "sensitized" to the allergen.

Then, the next time you're exposed to the allergen, your immune system kicks into action. The antibodies recognize it. That triggers the activation of special cells called mast cells. These cells are responsible for the symptoms in the lungs, skin, mucosal linings of the sinus and intestinal tract. These mast cells burst open, releasing something called histamines. You may have noticed allergy medications called anti-histamines. These are meant to block these histamines, thus alleviating the symptoms they cause. 

The amount of exposure matters as well. If you're allergic to, say, strawberries or avocado, you may not have a problem with a spoonful of guac or a berry or two. But then you eat a third berry, or another chip with guac, and you notice your eyes watering and itching, or perhaps hives raising on your arms, or even begin having difficulty taking a deep breath. This is because histamines cause inflammation. 

Sometimes you outgrow an allergy. And sometimes people don't react as strongly to the allergens that have given them problems in the past as they age. It's thought this is perhaps because our immune systems weaken as we age and therefore doesn't produce as strong of a response. But they never go away on their own. And some allergies will get worse over time...especially true with things like latex allergies and allergies to bee stings. There are also cross-affective allergies, where certain proteins shared by two separate allergens can cause a more severe reaction (for example, eating a banana during ragweed season). 

It's important to treat allergy symptoms, despite the temptation some have to "tough it out", since leaving them unchecked can cause more severe problems to develop (like asthma).