White blood cells are of different kinds. A type of white blood cell is Neutrophils which engulfs pathogens. These have lysosomes (which kills pathogens) There is a great video here – https://www.youtube.com/watch?v=0TvTyj5FAaQ
The method through which white blood cells engulf and break down pathogens is called phagocytosis. The cell membrane of the white blood cell surrounds the pathogens, and then traps it inside. The area in which it’s trapped is called a vesicle. Then the lysosomes attach to the vesicle, and release special proteins called enzymes, that digest (breakdown) the pathogen so it it no longer harmful.