White blood cells (WBC) or leukocytes are large, nucleated cells. They are much fewer in number than red blood cells. They are formed in the bone marrow and the lymph nodes of the armpits. White blood cells are an essential part of our body's immune system.

White blood cells are diverse and differ in size, number, nucleus structure and lifespan. Their lifespan varies from a few hours to several years.

Some white blood cells destroy microbes by engulfing them in, and others help by marking the pathogens so that they can be better recognised. White blood cells are also able to escape from the bloodstream into tissues.

Some white blood cells are converted into immunological memory cells. They quickly recognize and combat pathogens, and thus prevent diseases from re-emerging. This phenomenon is called immunity.

Immunity can also be obtained artificially by vaccination. In vaccination, antigens, certain molecules from the pathogen, are introduced into the body with an injection. This triggers an immune response.