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An immune system is a collection of mechanisms within an organism that protects against infection by identifying and killing pathogens and tumor cells. It detects a wide variety of pathogens, such as viruses and parasitic worms and distinguishes them from the organism's normal cells and tissues. Detection is complicated as pathogens adapt and evolve new ways to successfully infect the host organism.

To survive this challenge, several mechanisms have evolved that recognize and neutralize pathogens. Even simple unicellular organisms such as bacteria possess enzyme systems that protect against viral infections. Other basic immune mechanisms evolved in ancient eukaryotes and remain in their modern descendants, such as plants, fish, reptiles, and insects. These mechanisms include antimicrobial peptides called defensins, pattern recognition receptors, and the complement system. More sophisticated mechanisms, however, developed relatively recently, with the evolution of vertebrates.