K Cells, like natural killer cells, kill foreign invaders. However, unlike NK cells they require the help of an antibody made by B cells specific for the enemy cell. Both kinds of killer cells can slay on contact by means of a lethal burst of chemicals. Although lymphocytes are the main line of defense utilized by the immune system, they are by no means the only types of white blood cells that make up the total picture. But let me get a little more technical and bring in a couple of other terms that you may run across that can cause confusion.
Monocytes are phagocytes that circulate in the blood.
Kuppfer cells in the liver are an example of a macrophage. Activated by sensitized helper T cells, circulating monocytes are converted to macrophages, which are central in triggering the immune response. The macrophages engulf the foreign agent and highlight their antibodies so the helper T cell can quickly identify the invaders. Once the antibodies are marked by the macrophages as foreign invaders, helper T cells determine the immune response needed to neutralize or destroy the body's invader. Thus, the macrophage serves as a scout, providing the intelligence information with which the helper T cell determines subsequent immune response.