The adaptive immune system arose 500 million years ago in ectothermic (cold-blooded) vertebrates. Classically, the adaptive immune system has been defined by the presence of lymphocytes expressing recombination-activating gene (RAG)-dependent antigen receptors and the MHC. These features are found in all jawed vertebrates, including cartilaginous and bony fish, amphibians and reptiles and are most likely also found in the oldest class of jawed vertebrates, the extinct placoderms. However, with the discovery of an adaptive immune system in jawless fish based on an entirely different set of antigen receptors — the variable lymphocyte receptors — the divergence of T and B cells, and perhaps innate-like lymphocytes, goes back to the origin of all vertebrates. This Review explores how recent developments in comparative immunology have furthered our understanding of the origins and function of the adaptive immune system.

适应性免疫系统于五亿年前在变温（冷血）脊椎动物中出现。传统上，适应性免疫系统的定义是存在表达重组激活基因 (RAG) 依赖性抗原受体和 MHC 的淋巴细胞。这些特征存在于所有有颌脊椎动物中，包括软骨鱼和骨鱼、两栖动物和爬行动物，并且很可能也存在于最古老的有颌脊椎动物——已灭绝的盾皮鱼中。然而，随着在无颌鱼中发现基于一组完全不同的抗原受体（可变淋巴细胞受体）的适应性免疫系统，T 细胞和 B 细胞以及可能是先天性淋巴细胞的分歧可以追溯到所有免疫系统的起源。脊椎动物。本综述探讨了比较免疫学的最新发展如何进一步加深了我们对适应性免疫系统的起源和功能的理解。