<P> To complete development, immature B cells migrate from the bone marrow to the spleen as well as pass through two transitional stages: T1 and T2 . Throughout their migration to the spleen and after spleen entry, they are considered T1 B cells . Within the spleen, T1 B cells transition to T2 B cells . T2 B cells differentiate into either follicular (FO) B cells or marginal zone (MZ) B cells depending on signals received through the BCR and other receptors . Once differentiated, they are now considered mature B cells, or naive B cells . </P> <P> While immature and during the T1 phase, B cells express BCR of class IgH, but BCR expression changes to the classes IgM and IgD after transition into the T2 phase and while mature up to activation . </P> <P> B cell activation occurs in the secondary lymphoid organs (SLOs), such as the spleen and lymph nodes . After B cells mature in the bone marrow, they migrate through the blood to SLOs, which receive a constant supply of antigen through circulating lymph . At the SLO, B cell activation begins when the B cell binds to an antigen via its BCR . Of the three B cell subsets, FO B cells preferentially undergo T cell - dependent activation while MZ B cells and B1 B cells preferentially undergo T cell - independent activation . </P> <P> B cell activation is enhanced through the activity of CD21, a surface receptor in complex with surface proteins CD19 and CD81 (all three are collectively known as the B cell coreceptor complex). When a BCR binds an antigen tagged with a fragment of the C3 complement protein, CD21 binds the C3 fragment, co-ligates with the bound BCR, and signals are transduced through CD19 and CD81 to lower the activation threshold of the cell . </P>

Where are b cells and t cells located