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. Later these are carried away by blood and tend to reside in various parts of the lymphatic system, especially in the lymph nodes, thoracic duct and the spleen. Lymphocytes which are not processed in the thymus become B-cells. These pass through some unknown lymphoid tissue, where they are processed before they move on to the lymph glands. When blood or tissue is invaded, the various phagocytes, monocytes and neutrophils begin consuming the invaders - any type, dead or alive.