Surface immunoglobulin (Ig) is attached to the membrane of the effector B cells by its transmembrane region, while antibodies are the secreted form of Ig and lack the trans membrane region so that antibodies can be secreted into the bloodstream and body cavities.
As a result, surface Ig and antibodies are identical except for the transmembrane regions. Therefore, they are considered two forms of antibodies: soluble form or membrane-bound form (Parham 21-22).
The membrane-bound form of an antibody may be called a ''surface immunoglobulin'' (sIg) or a ''membrane immunoglobulin'' (mIg). It is part of the ''B cell receptor'' (BCR), which allows a B cell to detect when a specific antigen is present in the body and triggers B cell activation.
The BCR is composed of surface-bound IgD or IgM antibodies and associated Ig-α and Ig-β heterodimers, which are capable of signal transduction.
A typical human B cell will have 50,000 to 100,000 antibodies bound to its surface. In humans, the cell surface is bare around the B cell receptors for several thousand ångstroms, which further isolates the BCRs from competing influences.
Further Reading
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