From https://developer.mozilla.org/en-US/docs/Glossary/Entity
HTML has a set of four reserved characters that must be represented as
entities. All other characters can be represented with entities as
needed, but it is not mandatory. These are the entities used for those
four reserved characters :
& (&) – Required, so that a browser does not misinterpret it as the beginning of an entity.
< (<) - Required because otherwise a browser could misinterpret it as the beginning of a tag
> (>) – Required because otherwise a browser could misinterpret it as the ending of a tag
" (“) – Required only within attributes value surrender by quote sign
See here for a full list of all HTML character entities – https://dev.w3.org/html5/html-author/charref
The character ‘&’ is reserved word which has special purpose. That’s why you have to use it in specific way.
Here you can find the HTML Entities. http://www.w3schools.com/html/html_entities.asp