Our immune system is designed to protect us from danger. Upon pathogen invasion and tissue injury, activation of both innate and adaptive immunity enables us to combat infection and to repair tissue damage. Tenascin-C is a large, extracellular matrix glycoprotein that has a very tightly controlled pattern of expression. Little or no tenascin-C is expressed in most healthy adult tissues; however, it is rapidly and transiently induced at sites of tissue injury and infection. Persistent tenascin-C expression is associated with pathologies such as chronic, non-healing wounds, autoimmune diseases, cancer, and fibrotic diseases. We discuss the myriad roles that this multifunctional molecule plays during the immune response, with a focus on how tissue levels of tenascin-C are regulated and the consequences of misregulated tenascin-C expression in immune regulated disease pathogenesis.