Victoria Picking answered
White blood cells have adapted so that they can perform their function in the best way possible.
The function of a white blood cell is get rid of any harmful micro-organisms in the body before they cause any damage to the body.
The cell membrane has also been designed to be very sensitive. It has to be so that it can easily detect and foreign micro-organisms in the body.
In order to destroy the micro-organism, it needs to be engulfed into the white blood cell, so that the enzymes that are inside it can start to digest the foreign bodies.