<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> <P> Antigens that activate B cells with the help of T - cell are known as T cell - dependent (TD) antigens and include foreign proteins . They are named as such because they are unable to induce a humoral response in organisms that lack T cells . B cell response to these antigens takes multiple days, though antibodies generated have a higher affinity and are more functionally versatile than those generated from T cell - independent activation . </P>

Where do b cells mature and what has to happen for them to mature