Immature B cells are precursors of B cells that have a critical role in prevention of auto-immune responses. In most mammals, they are produced in the bone marrow, where they are tested for auto-reactivity, and only then migrate to secondary lymphoid tissues (e.g. spleen, lymph nodes, etc.), where some of them differentiate into mature B cells. The immature B cells develops following thesuccessful recombination of the IgL locus to produce the IgM receptor that is essential for subsequent B cell function at the pre-B cell stage.