There are five different types of leukocytes: three types of granulocytes (eosinophil, basophil, and neutrophil) and two types of agranulocytes (monocyte and lymphocyte).
NK cells are large lymphocytes with granules and they target cells infected with intracellular organisms, like viruses, as well as cells that pose a threat like cancer cells.
When something like that happens, cells are supposed to undergo apoptosis - or programmed cell death, but instead the lymphocyte starts to divide uncontrollably - becoming a neoplastic cell.
When looking at the blood vessels in particular, the classic finding is seeing lymphocytes in the walls of capillaries, veins, and arteries of all sizes, making them inflamed and boggy.