How does the bone protect the immune system?

please help

The bone marrow(which is found in bones) produces cells called lymphocytes which is an essential part to the immune system. The lymphocytes circulate in your blood, and there are 2 types of lymphocytes: T and B cells. The B cells produce molecules that destroy viruses or bacteria

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.