Transitional B cells are B cells at an intermediate stage in their development between bone marrow immature cells and mature B cells in the spleen. Primary B cell development takes place in the bone marrow, where immature B cells must generate a functional B cell receptor (BCR) and overcome negative selection induced by reactivity with autoantigens.[1] Transitional cells can be found in the bone marrow, peripheral blood, and spleen, and only a fraction of the immature B cells that survive after the transitional stage become mature B cells in secondary lymphoid organs such as the spleen.