Question

how do some human white blood cells help destroy pathogenic bacteria

Answer 1

it is their primary functions . For eg. leucocytes.

Answer 2

A unique while blood cells are called lymphocyte.

These lymphocytes can identify the antigen as foreign body and it produces antibody that is specific to that antigen.  

These antibodies can destroy the pathogen.  

These white blood cells can also generate chemicals known as antitoxin.

For example, Tetanus, diphtheria, and scarlet fever related bacteria secrete toxins, which can be destroyed by the immune system of body made of white blood cells.