β2-integrins in Immunity: New Roles for Old Players

β2-integrins are adhesion- and mechanoreceptors expressed exclusively on leukocytes. They mediate diverse functions in immunity and are of fundamental importance in processes such as leukocyte trafficking, phagocytosis, T cell and B cell activation and differentiation and T cell-mediated killing. Furthermore, they can mediate immunosuppressive signaling in cells such as macrophages and dendritic cells. Their crucial importance for immune system function is illustrated by the rare but devastating genetic immunodeficiency disorders Leukocyte adhesion deficiency type I and type III, where β2-integrin expression or function is reduced or lost. Here, we discuss the structure of β2-integrins and their various ligand-binding properties. Furthermore, we describe their regulation, signaling, and mechanotransduction properties in immune cells. In addition, we describe their various functional roles in different immune cells, in host defense, infection, inflammatory diseases, and cancer.