B cell receptor (BCR) is a membrane-bound immunoglobulin molecule found attached to the surface of B cells. Every B cell has approximately 105 B cell receptors in itsplasma membrane. Each BCR is composed of 4 chains, 2 identical heavy chains and 2 identical light chains. The chains are joined by disulfide bonds. B cell receptors act as the first line of defense against invading pathogens and are an essential part of the immune system. Their main role is recognizing and binding specific foreign substances known as antigens. On binding with an antigen, the B cell receptor stimulates B cells to differentiate into antibody-secreting cells. B cell receptors have a complex signaling system that results in several distinct outcomes such as tolerance, survival, proliferation, or apoptosis, depending on the circumstances.