Figure 1. Compartmentalization of adaptive immunity in vertebrates.

a, In jawed vertebrates, a haematopoietic stem cell differentiates into a common lymphoid progenitor, and then into three lineages: T cells, which are responsible for the cellular immune response; B cells, responsible for the humoral response; and natural killer cells (not shown). On T cells, the receptor is the T-cell antigen-binding receptor and on B cells it is membrane-bound immunoglobulin. Alternative RNA processing changes membrane-bound immunoglobulin to a secreted antibody of the same specificity in a plasma cell. b, In jawless vertebrates, an as-yet-undefined precursor is thought to give rise to two lineages of lymphocyte-like cells defined by their expression of variable lymphocyte receptor (VLR) A or B. As Guo et al.¹ show in the lamprey, VLRA is expressed only on the cell surface; VLRB is expressed initially on the cell surface and is also secreted. The molecular mechanisms by which adaptive immunity is compartmentalized into cellular and humoral responses in jawed and jawless vertebrates are fundamentally different. But in both cases there was evidently evolutionary pressure to create specific lineage-restricted immune systems.