General and Emerging Concepts of Immunity

Immunity is essential if multicellular organisms are to survive in environments where many other organisms (typically former free-living unicellular forms of life that became parasites) live that may invade and compromise organismal homeostasis. It is therefore not surprising that a dedicated defensive system has evolved in essentially every living organism to fend off external threats. Two major cellular systems have evolved in mammals for protection: an innate system that ensures a rapid response, which can in some instances be too aggressive as it lacks specificity and (was believed to) lack(s) memory; and an adaptive system, more sophisticated system that ensures high specificity and long-term memory. This classical view of our immune system is however being actively reevaluated through the use of new tools available for biomedical research, including single cell technologies. Indeed, surprising findings over the past decade are reshaping this functional division, refining the number and classification of immune cells, and altogether challenging the classic architecture and purpose of the immune system. Here, we review basic and emerging concepts of immunity through the prism of the classical division of the immune system into innate and adaptive, but provide links to new concepts and relevance in human disease.