Why do white blood cells need to change shape in order to accomplish their function

Well, According to 

BBC - GCSE Bitesize: Blood,
 White blood cells can engulf bacteria and other pathogens by phagocytosis. They can change shape easily and produce enzymes that digest the pathogens. Other types of white blood cell secrete antibodies that help destroy pathogens.

There are very interesting facts, and stories to that site, Plus, I am not Copying and pasting,  If I ever do, I tell you the site to where it Found your answer too. :)

To produce enzymes that digest pathogens

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