A B cell is a class of lymphocyte (white blood cell) that secretes antibodies against antigens. B cells perform the job of immune surveillance. They begin to produce antibodies when they are fully activated by an antigen (foreign bacterial or viral substance). Each B cell has a special protein wrapping around its surface. This protein is a membrane-bound immunoglobulin that functions as a receptor, which is referred to as the B cell receptor, or BCR.
The B cell receptor will bind to a foreign antigen and activate the B cell, which, in turn, engulfs the bound antigen molecule by receptor-mediated endocytosis. The antigen is destroyed and its fragments are displayed on the cell surface. Then, T helper cells bind the B cell and secrete lymphokines which stimulate the B cell to begin a cell cycle and, through repeated mitosis, develop into a clone of cells with identical B cell receptor (BCR).
Immature B cells are produced in the bone marrow. Then these immature B cells migrate to the spleen, where they are called transitional B cells, and some of these cells differentiate into mature B lymphocytes.