Taster Resource

B cells begin as stem cells within the bone marrow. They then differentiate into pro-B cells, which become pre-B cells once their receptors begin to become expressed. Once the cells express all their BCRs, and other surface molecules they are known as immature naive B cells. They become mature only when they leave the bone marrow, and enter the secondary lymphoid tissues (lymph nodes, spleen, etc) - they remain naive until they bind to 'their' specific antigen.

Each B cell has one unique specificity (thus each is different).

Once the B cell encounters its specific antigen, it becomes 'activated' (often T cells help activate the B cells, in the form of th2 cells which provide a co stimulatory signal to fully activate the B cell). After activation, the B cell differentiates into either a memory cell, or more commonly, a plasma cell. A plasma cell is essentially a 'factory' for making antibodies. The antibodies are a soluble form of the BCR, and thus have the same specificity as the BCR. B cells can each produce different classes of antibody that have different effector functions.

As mentioned, each B cell has a unique specificity - hence finding a B cell with a particular specificity is rare. When a BCR does encounter its antigen, the B cell undergoes rapid clonal expansion (due to effects of the cytokine IL2 which promotes B cell proliferation). Hence there are a larger number of the required B cells to 'deal' with the pathogen.

Antibody structure

An antibody is made of two identical heavy chains, and two identical light chains. The two chains are held together by disulfide bonds and non-covalent interactions there are five types of immunoglobulin heavy chain - alpha, sigma, epsilon, gamma, and mu. The five different types of heavy chain correspond to the 5 different antibody classes, or isotypes. There are also two light chains, known as lambda, or kappa. The constant region of the antibody defines the effector function, and the variable region defines specificity. Each antibody (and thus each BCR) has a unique specificity.