Are multi-celled organisms made of single celled organisms?

I have been pondering this question for a long time and I have arrived to the answer: yes

Eg: white blood cells are only in symbiotic relationship with the human body... the body houses the cell eats the bacteria in the body... your brain doesn't control the white cell, it eats so that it survives

MORE INFO: you are only made up of simple atoms grouped together and humans only have jobs and stuff to help it's own species and nothing else.... we are just like any other organism.... just a lot smarter, the only difference