While a T cell receptor resembles just the upper arm of an antibody molecule where the variable regions can be found, the B cell receptor closely resembles the whole antibody molecule. In fact, the B cell receptor is also known as membrane immunoglobulin (mIg, generally IgD or IgM) as it is encoded by the same genes that code for antibodies (immunoglobulins). These variable regions found on the outer surface of the receptor are what gives rise to the wide array of potential binding specificities a B cell may demonstrate.

Clonal expansion

Once the mature B cell matches a pathogen or other antigen it will begin to proliferate. This produces many identical cells with identical binding specificity in a process called 'clonal expansion'. Some of these daughter cells will also differentiate into plasma cells. These important effector cells produce and secrete a soluble version of the original B cell's membrane.