In the following question, sentences of a paragraph have been jumbled and labelled as A, B, C, and D. You are required to rearrange the jumbled sentences of the paragraph and mark your response accordingly by selecting the correct option.

A: Any foreign substance in your body that makes the immune system attack it, is called an antigen.

B: It is called the immune system and it knows when there is something inside your body that should not be there.

C: Therefore this special system protects you from illness and diseases caused by antigens.

D: You have your own little army of cells inside you, always fighting to keep you healthy.