White blood cells are cells adapted to combat against infectious disease and any other foreign material that may enter the body. They are adapted to change shape, allowing them to wrap around microbes in the body and engulf them. Once microbes are engulfed they can be digested by the white blood cell and this helps the immune system defend the body from further attack.
The ability of white blood cells to change shape also allows them to squeeze through capillary walls to reach microbes in tissues and organs.
White blood cells come in different forms, the two most common being phagocytes and lymphocytes.
Phagocytes are different to lymphocytes in that they have a much smaller, multi-lobed nucleus and destroy bacteria by a process called phagocytosis, where the bacterium is engulfed by the cell whilst enzymes are secreted to break down the bacterium.
Lymphocytes produce antibodies that bind to specific antigens on foreign cells and uncommon substances. Antibodies cause agglutination (clumping together) of such cells/substances, so phagocytes can easily engulf and destroy them by phagocytosis.
Lymphocytes have large nuclei and a much smaller amount of cytoplasm compared to other cells.