Trauma, bacteria and dirt signal to white cells in nearby vessels there is damage to control. Neutrophils, the most active and phagocytic of the white blood cells, become sticky and begin to adhere to the inside of the vessel wall. Adherence slows the cells down, making them "roll" on the inside of the vessel. The neutrophils then become superadherent and squeeze out between endothelial cells that line the vessel, a phenomenon called "diapedesis".
From there, the white cells must FIND, EAT, and KILL foreign microbes.
CHEMOTAXIS ... how white cells FIND the bad guys
PHAGOCYTOSIS ... how white cells EAT microbes
OXIDATIVE BURST ... how white cells KILL bacteria