As mentioned before, each B cell is comitted to the production of antibody expressing a unique VH-VL combination, and the surface and excreted immunoglobulin are the same.These observations form the basis of Burnet's clonal selection theory in that each B cell expresses a surface immunoglobulin that acts as its antigen-receptor site. Contact with the antigen and helper T-cell factors commit each B cell to divide and differentiate to produce more of the same VH-VL antibody. A number of these B cells become memory cells so that a greater number of antigen specific B cells are available on a secondary contact with the same antigen. This phenomenon is known as clonal expansion and helps to account for the greater secondary response.
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