White blood cells produce a proteins which are called Antibodies, they are made when the white blood cell finds and unknown bacteria which they have never seen before this is called a Antigen when they find an Antigen the white blood cell will let the Antibodies they made lock on to the Antigen so when the come across it the white blood cells will destroy it. After they destroy that Antigen the Antibodies start producing rapidly so that they can explore the body for similar bacteria or viruses. Id the person is infected with the same pathogen again the white blood cell will produce that antibody again so that they can destroy it quickly and that is how people are immune from that bacteria or virus and will not become ill from it.

Antibodies are produced to only stick to one specific Antigen it will only lock on to that and nothing else this is why the Antibodies are produced by white blood cells because they can find different diseases and then destroy them before they cause any harm to the Human body.