White blood cells, called leukocytes, are outnumbered by red blood cells by about 600 to 1. The white cells are made up of an army of bodily defenders which die in the billions as they battle infections and disease. White cells are complete within themselves, with internal structure and nuclei, so they are ready to go anywhere in the body at a moment's notice.
Leukocytes are specialists.
Half of them patrol the blood while the rest are surveying the tissues to see that all is well in that area. White cells can creep along capillaries, squeeze through gaps in the blood cell walls and between spaces of the tissue cells. Bacteria, virus, fungi and parasites are constantly sneaking in through cuts in the skin or into the linings in the respiratory and digestive tracts. Lymphocytes originate from the bone marrow. These become specialized as they are released into the blood stream.