Frames of reference for TCR diversity measurements. (A-C) In this schematic, circles represent T cells, their shading represents their phenotype (eg, naive CD4⁺ T cells), and their letter represents their TCR clonotype. (A) Total body TCR diversity (ie, unique TCR sequences in the entire body). (B) TCR diversity per fixed volume of blood or tissue (eg, unique TCR sequences per 1.5 mL of whole blood). (C) TCR diversity per fixed number of cells, often of a defined subpopulation (eg, number of unique TCR sequences found in a sample of 1.2 million naive CD4⁺ cells).