B-cells are originate and mature in the bone marrow. VDJ recombination occurs during the early development of B-cells. The light and heavy chains are selected by gene rearrangement and they together form the B-cell receptors.
B-cell receptors are expressed on the cell membranes of the B cells.
B Cell Receptors (BCRs)
B-cell receptors are also known as membrane-bound immunoglobulins. This is because their structure is almost identical to the secreted antibodies, one difference is that B-cell receptors have an extra protein sequence at the C terminal of each heavy chain. This sequence anchors B cell receptors to the B cell membrane.
It consists of a transmembrane domain and a cytoplasmic domain. 'That these extra domains are absent in the secreted antibodies'.
Each mature naive B-cell Express membrane bound immunoglobulin M (Ig M) and membrane-bound immunoglobulin D (Ig D) as its B-cell receptors. Both of these receptors are of same antigenic specificity. This means both recognise the same epitope on an antigen.
• The function of these B-cell receptors is to recognise and bind the specific antigen.
For B-cell activation the nucleus of the b-cell should get the signal that specific antigen has been recognised and bound by the B-cell receptors.
• The cytoplasmic tails of B-cell receptors are very short.
• They cannot convey antigen recognition signal to the nucleus by themselves.
Signal to the nucleus is conveyed by two accessory proteins, which are associated with these B-cell receptors. These accessory proteins are Ig α (alpha) and Ig β (beta).
• Together they form a heterodimer held together by a disulfide bond.
• Ig α & Ig β chains are glycoproteins and they are transmembrane molecules.
Their extracellular portions consist of single Ig like domain. this heterodimer conveys the intracellular signals, indicating that antigen has been found.
• Ig α and Ig β molecules are expressed only in B-cells. the designation IgE in these accessory proteins represents that these proteins are associated with the immunoglobulins.
B-cell receptor complex is made up of a membrane bound immunoglobulin and Ig α and Ig β heterodimer.
• Antigen recognition and binding is performed by the membrane bound immunoglobulin that is the B-cell receptor.
• Signal to the nucleus is conveyed by heterodimer Ig α and Ig β
Another important requirement of B-cell to generate the activation signal is that
- Multiple B-cell receptors must be brought close together on the surface of the B cell.
Antigen generally has multiple copies of the same epitope on its surface. Each epitope can bind adjacent B-cell receptors on one B-cell. Once bound surface b-cell receptors begin to cluster. This process activates the accessory proteins like Ig α and Ig β and initiate the signaling process.
