White blood cells (also called leukocytes) are a key part of the body's system for defending itself against infection. They can move in and out of the bloodstream to reach affected tissues. Blood contains far fewer WBCs than red blood cells, although the body can increase WBC production to fight infection. There are several types of WBCs, and their life spans vary from a few days to months. New cells are constantly being formed in the bone marrow.
Several different parts of blood are involved in fighting infection. White blood cells called granulocytes and lymphocytes travel along the walls of blood vessels. They fight germs such as bacteria and viruses and may also attempt to destroy cells that have become infected or have changed into cancer cells.
Certain types of WBCs produce antibodies, special proteins that recognize foreign materials and help the body destroy or neutralize them. The white cell count (the number of cells in a given amount of blood) in someone with an infection often is higher than usual because more WBCs are being produced or are entering the bloodstream to battle the infection.
After the body has been challenged by some infections, lymphocytes "remember" how to make the specific antibodies that will quickly attack the same germ if it enters the body again
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