Leukocytes or white blood cells destroy abnormal cells and also provide protection against bacteria and other foreign matter.  These interactions are transitory in nature but are crucial as an immediate immune response. To fight infection, leukocytes must move from the blood into the affected tissues.  This movement into tissues is called extravasation.  It requires successive forming and breaking of cell-cell interactions between the leukocytes and the endothelial cells that line blood vessels. These cell-cell interactions are mediated mainly by a group of Cell Adhesion Molecules (CAMs) called selectins.
T helper cells, central to the immune system, interact with other leukocytes by releasing signals known as cytokines which activate and stimulate the proliferation of B cells and killer T cells. T helper cells also directly interact with macrophages, cells that engulf foreign matter and display antigens on its surface.  T-helper cells that possess the appropriate receptors can bind to these antigens and proliferate resulting in T-helper cells that have the ability to identify the same antigens.