The B-cell receptor is a transmembrane receptor protein

Receptor (biochemistry)

In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...

 located on the outer surface

Cell membrane

The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...

 of B-cells. The receptor's binding moiety is composed of a membrane-bound antibody

Antibody

An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...

 that, like all antibodies, has a unique and randomly-determined antigen

Antigen

An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...

-binding site. When a B-cell is activated by its first encounter with an antigen that binds to its receptor (its "cognate antigen"), the cell proliferates and differentiates to generate a population of antibody-secreting plasma B cells and memory B cell

Memory B cell

Memory B cells are a B cell sub-type that are formed following primary infection.-Primary response, paratopes, and epitopes:In wake of first infection involving a particular antigen, the responding naïve cells proliferate to produce a colony of cells, most of which differentiate into the plasma...

s.

Components of the B-cell receptor

The B-cell receptor is composed of two parts:

1. Ligand binding moiety: A membrane-bound bound immunoglobulin molecule of one isotype (IgD, IgM, IgE or IgE). With the exception of the presence of an integral membrane
   Integral membrane protein
   An integral membrane protein is a protein molecule that is permanently attached to the biological membrane. Proteins that cross the membrane are surrounded by "annular" lipids, which are defined as lipids that are in direct contact with a membrane protein...
   
    domain, these are identical to their secreted forms.
2. Signal transduction moiety: A heterodimer called Ig-α
   CD79A
   CD79a molecule, immunoglobulin-associated alpha, also known as mb-1, is a human gene.The mb-1 gene codes for a phosphoprotein, designated CD79a CD79a molecule, immunoglobulin-associated alpha, also known as mb-1, is a human gene.The mb-1 gene codes for a phosphoprotein, designated CD79a CD79a...
   
   /Ig-β
   CD79B
   CD79b molecule, immunoglobulin-associated beta, also known as CD79B , is a human gene.It is associated with agammaglobulinemia-6.-External links:...
   
    (CD79
   CD79
   CD79 is a transmembrane protein that forms a complex with the B-cell receptor and generates a signal following recognition of antigen by the BCR...
   
   ), bound together by disulfide bridges
   Disulfide bond
   In chemistry, a disulfide bond is a covalent bond, usually derived by the coupling of two thiol groups. The linkage is also called an SS-bond or disulfide bridge. The overall connectivity is therefore R-S-S-R. The terminology is widely used in biochemistry...
   
   . Each member of the dimer spans the plasma membrane and has a cytoplasmic tail bearing an immunoreceptor tyrosine-based activation motif (ITAM)
   Immunoreceptor tyrosine-based activation motif
   An immunoreceptor tyrosine-based activation motif is a conserved sequence of four amino acids that is repeated twice in the cytoplasmic tails of certain cell surface proteins of the immune system....
   
   .