B cells are one of the two main types of lymphocytes. As opposed to T cells, B cells play a pivotal role in the humoral immune response, which is an antibody-mediated immune response. Each B-cell receptor (BCR) which sits on their cell surface, is specific for a particular antigen. The mature B cell expresses both the IgM and the IgD. The BCR is critical for B cell function, toward production of antibodies, as well as for their functioning as antigen-presenting cells (APCs). A mature B cell can differentiate into several types of B cells, the most common being plasma and memory B cells.