1 reason: Certain white blood cells called lymphocytes are responsible for producing the antibodies which destroy specific pathogens by detecting antigens (foreign chemicals released by the pathogen).
2 reason: Pathogens can produce harmful chemicals called toxins, and white blood cells produce antitoxins to destroy these.
3 reason: White blood cells are key in protecting against pathogens. They ingest the harmful microorganisms (they surround them and take them up - a bit like eating them, but you must use the word ingest).
I found these on internet and a little bit of my knowledge but I hope this helps :)