Updated publication reference for PubMed record(s): 28467900. Different pathogens trigger naïve T cells to express distinct sets of effector proteins. To better understand the molecular mechanisms that drive this functional specification, we used high resolution, label-free mass spectrometry to measure proteomic differences between the seven largest circulating human CD8+ T cell subsets. Unsupervised hierarchical clustering of the proteomes placed naïve and CD45RA-expressing effector-type T cells at the extremes of the spectrum with central-memory and other effector-memory stages located in between. Prominent differences between the subsets included expression of various granzymes, signaling proteins and molecules involved in metabolic regulation. Remarkably, whereas most of the proteomic changes between the subsets were gradual, a small proportion of proteins were regulated only in discrete subsets. The data obtained from this proteome analysis correspond best to a progressive differentiation model in which specific stable traits are gradually acquired during pathogen-specific development.