The immune system consists of a great variety of cell types fulfilling diverse tasks in monitoring tissue homeostasis to protect against pathogens and to remove damaged cells. To ensure the smooth, controlled function of this highly complex system, immune cells use a wide range of biochemical signaling pathways activated by soluble proteins or direct cell-cell contacts. Those pathways are also targeted by modern drugs, for instance, cancer immunotherapeutic agents, that direct the immune response against specific structures or cell types.