B cells or B lymphocytes develop from hematopoietic stem cells in the bone marrow and are involved in the humoral response. The B-cell receptor is a membrane-bound immunoglobulin. There are approximately 105 B cell receptors on the plasma membrane surface. The receptor of the B cell binds an antigen and delivers this inside the cell in a vesicle where the antigen undergoes processing and is then presented on the cell surface as a complex with an MHC class 2 molecule. CD4+ T cells then respond to this antigen presentation by releasing cytokines leading to B cell proliferation and differentiation. B cells differentiate into plasma memory cells and antibody-secreting cells .