This chapter describes how the nascent science of immunity was a chemical immunology, the science of antibodies. It remained so for half a century before it became a cellular immunology, the science of lymphocytes, after the Second World War. When critical cells and molecules were identified, they were integrated into a biological system dedicated to the defense of the organism, which received the name “immune system” in the early 1960s. Until it was discovered in the early 1990s that we have not one but two immune systems, an innate immune system capable of perceiving “danger signals” and an adaptive immune system capable of perceiving “self and non-self”.