system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Vitamins; what are they? How do they work? What processes in the body require them and how?

We're always being told certain things contain vitamins and that those are good for one's health, but what actually are they? What functions of the body require them, and how does it make use of them? 

Bonus round: Antioxidants; another health food buzzword - what are they, and what do they do?
gpt
A "vitamin," generally, is any of a group of groups of molecules, each group of molecules constituting one vitamin. For example, there are eight compounds that constitute vitamin E.

What constitutes a vitamin is that it can't be something the body makes on its own: we must take it in through our diet.

Here are the thirteen such groups that our bodies require, along with a non-exhaustive list of what our bodies need them for:

1. Vitamin A (used in eyesight, skin maintenance, human development)

2. The B complex (1, 2, 3, 5, 6, 7, 9, and 12) (used in cell metabolism)

3. Vitamin C (Used in immune response)

4. Vitamin D (controls absorption of calcium, magnesium, and phosphate in our gut)

5. Vitamin E (we're not sure quite what it does, but it may have antioxidant functions, while I'll get to)

6. Vitamin K (blood clotting, bone formation, etc)

Our bodies use these compounds in various ways: reaction mediation, breaking them down to use their components, etc.

#Bonus Question:

Antioxidants. As the name implies, they inhibit a class of reactions called "oxidation reactions," which is the same reaction that forms rust. In the body, instead of rust, they create something called a "free radical." A radical in general is anything with a single unpaired electron. Because of that, they are highly reactive. "Free" radicals are uncontrolled by the body, and they can inflict damage to us.