What is the main function of the white blood cell

to fight viruses and bacteria

White blood cells are like the police of your blood stream. They detect what is going on in the blood and create antibodies to fight off any intruders. A high number of white blood cells means there is something invading. But too few white blood cells could be: (www.mayoclinic:)

A low white blood cell count usually is caused by one of the following:

Viral infections that temporarily disrupt bone marrow function
Congenital disorders characterized by diminished bone marrow function
Cancer or other diseases that damage bone marrow
Autoimmune disorders that destroy white blood cells or bone marrow cells
Overwhelming infections that use up white blood cells faster than they can be produced
Drugs that destroy white blood cells or damage bone marrow

White blood cells or 'neutrophils' engulf and digest foreign pathogens using the digestive enzymes contained inside them. They are phagocytes and part of the immune response.

to kill bacteria or pathogens.
they produce antibodies, antitoxins or injest pathogens/disease causing microorganisms.

To help the body fight disease and protect the body from viruses, etc.