An Overview of the B-Cell and Immunoglobulin
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An Overview of the B-Cell and Immunoglobulin

This article relays information about B-cells, their functions and a general overview of the B-cell receptor, immunoglobulin, and how it functions to defend the body in the primary adaptive immune response to a pathogen or invader. B-cells are extremely important to the immune system and are a cornerstone to the study of immunology.

B-cells are lymphocytes that originate from the lymphoid lineage of the hematopoietic stem cells.  These B-cells are a part of the humoral immune response and are among the first cells that react to a primary immune response.  They are generated in the bone marrow and also complete their maturation phase there.  The primary function of the B-cell is to produce antibodies.

B-cells react to pathogens by specific receptors on their surface, known as B-cell receptors. The B-cell receptors are also known as immunoglobulins and are specific to the antigen they bind, meaning they can bind to one type of antigen and one antigen only.  Immunoglobulin is found both on the membrane of the B-cell but it is also present in secreted form.  When in the secreted form, it is known as an antibody.

Immunoglobulin, commonly abbreviated Ig, has 4 polypeptide chains.  They consist of two heavy chains and two light chains.  They are constructed in a Y-shape with the upper two prongs of the Y being the amino region and the base of the Y being the carboxyl region.  The amino region is commonly known as the variable region and the carboxyl region as the constant region.

The constant region (the heavy chain segment) contains five different specific types of Ig.  They are IgG, IgM, IgD, IgA, and IgE.  When referring to a specific isotype of Ig, one uses the Greek letter associated with their name: g, m, d, a, e, respectively.  The variable region(the light chain) contains only two different isotypes which are the kappa(k) and lambda(l) regions.  Only one type of Ig is on a B-cell, save for naïve B-cells, which contain IgM and IgD, and either a kappa or a lambda light chain, but never both. 

IgM is the first antibody produced when a pathogen is invading the body.  IgM also has an effector function in which it can change its isotype based on the specific pathogen involved.  IgG is the most abundant type of immunoglobulin and has effector functioning capabilities like the IgM.  IgG also has the distinction of being transferred across the placenta to the fetus so that the fetus will be protected in case of infection.  IgA is the main antibody associated with the mucus and mucosal surfaces, such as saliva and sweat.  IgA is also present in defense against the allergens commonly found in the atmosphere.  IgD is the antibody associated with IgM when produced on naïve B-cells.  IgE antibodies are specialized towards binding with mast cells and protecting against parasites.

B-cells are very important to the humoral immune response and in the production of antibodies, also known as immunoglobulins.  Immunoglobulins are found in 5 main types and have different functioning capabilities based on those types.  Because of this variety, these B-cells and immunoglobulins are extremely effective in their defense of the human body.

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