A B cell arises from haemopoietic stem cells (blood forming stem cells) and matures in the bone marrow.
The B cell antigen-specific receptor is an antibody molecule on the B cell surface, and recognizes whole pathogens without any need for antigen processing.
Each B cell is specific for a particular epitope, or site, on a foreign protein or carbohydrate. Each lineage of B cell expresses a different antibody, so the complete set of B cell antigen receptors (BCR's) represent all the antibodies that the body can manufacture.
Each B cell is programmed to make one specific antibody.
For example, one B cell will make an antibody that blocks a virus that causes the common cold, while another produces an antibody that attacks a bacterium that causes pneumonia.
If a particular BCRs binds to the a bacterial antigen, the antigen molecules will be engulfed into the B cell by a process called receptor-mediated endocytosis.
The antigen is then broken down into peptide fragments. These peptide ragments are then displayed at the cells surface in the grove of a molecule called Class II MHC. ( see genes)
Helper T cells with a T cell receptor that recognizes the displayed peptide fragment binds to the B cell, and releases lymphokines (a type of cytokine) that stimulate the B cells to proliferate, differentiate, and become an antibody producing cell.
The interaction with T cells maintains the specificity of the B cell response and also leads to the switch of antibodies from the IgM isotype to the more efficient IgG isotype. (see antibodies)
The activated B cell then begins to divide into two, producing plasma cells, which secrete immunoglobulins (antibodies), and Memory Bcells. (see Lymphocytes,B&Tcells)
Plasma Cells
When a B cell encounters its triggering antigen, it gives rise to many large cells known as plasma cells. Every plasma cell is essentially a factory for producing an antibody. Each of the plasma cells descended from a given B cell manufactures millions of identical antibody molecules and pours them into the bloodstream.The plasma cells secrete millions of copies of the antibody (immunoglobulin)that recognizes a specific antigen. B cells are the only cells in the body that produce antibodies. They then enter the blood stream and circulate through the body.
These antibodies can :-
1) circulate in blood plasma and lymph
2)bind to pathogens expressing the antigen marking them for destruction by complement activation, (see Complement System) or for uptake and destruction by phagocytes.
3) Neutralize challenges directly, by binding to bacterial toxins
4) interfering with the receptors that viruses and bacteria use to infect cells. (see Antibodies Below)
Memory cells
When B cells and T cells are activated and begin to replicate, some of their offspring will become long-lived memory cells. Throughout a life time , these memory cells will remember each specific pathogen encountered and can mount a strong response if the pathogen is detected again. This is "adaptive immunity" because it occurs during the lifetime of an individual as an adaptation to infection with that pathogen, and prepares the immune system for future challenges. Immunological memory can either be in the form of passive short-term memory or active long-term memory .
Antibodies
Antibodies are (Immunoglobulins, abbreviated Ig) and are also any of the structurally related glycoproteins. Immunoglobulins (antibodies) are found in serum and in secretions from mucosal surfaces.
They are produced and secreted by plasma cells which are found mainly within lymph nodes, and which do not circulate. Plasma cells are derived from B lymphocytes: (see Bcells) An immunoglobulin (Ig) molecule (an antibody) consists of two light polypeptide chains and two heavy polypeptide chains. A polypeptide chain is the structural element of a protein, consisting of a series of amino acid residues , known as peptides, joined together by peptide bonds.
A Heavy chain, known as the H chain, is any of the large polypeptide chains of five isotypes, (classes eg: IgG, IgA. see isotypes) that, paired with the light chains, make up the antibody molecule.
Heavy chains bear the antigenic determinants that differentiate the immunoglobulin classes. (IgA, IgG etc.) A light chain, is known as an L chain. Light chains exist in two classes, lambda and kappa. Each antibody molecule has either lambda or kappa light chains, not both, and are unrelated to immunoglobulin class differences.
An L chain is either one of the two small polypeptide chains that, when linked to heavy chains by disulfide bonds, make up the antibody molecule. (as noted above with the H chain.) (see D & J links in Genes)
Antibodies are unique molecules, derived from the 'immunoglobulin supergene'.
One end of the Ig binds to antigens known as the Fab portion,( so called because it is the Fragment of the molecule which is antigen binding), and the other end which is Crystallizable, and therefore called Fc, is responsible for "effector" functions. (meaning having the capability in altering the activity of a molecule)
The regions on the Immunoglobulin molecule concerned with antigen binding, are extremely variable, whereas in other regions of the molecule, they are relatively constant. Thus, each heavy chain and each light chain possess a variable and a constant region. (see under Genes)
IgA exists in monomeric (a single segment) and a dimeric form,( combination of two identical molecules.)
IgA molecules also receive a secretory component from the epithelial cells into which they pass. This is used to transport them through the cell and remains attached to the IgA molecule, within secretions at the mucosal surface.
An IgM molecule is in pentameric form,(a molecule composed of approx. 5 linked monomers (single) units ) Both IgA and IgM are polymers. ( A polymer is a naturally occurring compound, consisting of large molecules made up of a linked series of repeated simple monomers ) The links between the "monomers" are made by a J polypeptide chain. (see genes)
L=light chains are separated from H=heavy chains by disulphide (S-S) links. Intrachain S-S links divide H and L chains into domains which are separately folded. Thus, an IgG molecule contains 3 H heavy chain domains, written CH1, CH2 and CH3. Between CH1 and CH2, there are many cysteine and proline residues. This is known as the hinge region and confers flexibility to the Fab arms of the Ig molecule, which is used when antibody interacts with antigen. (see under genes)
Isotypes
The Isotype of an immunoglobulin is determined by the constant region. (See genes) Isotypes are antigenic determinants that characterize classes and subclasses of heavy chains and types and subtypes of light chains.
Both Heavy and light chains are found on the constant region of a molecule, but Heavy chain classes of immunoglobulin are located on the Fc portion of the constant region of the molecule. These are responsible for effecting functions of a cell , meaning that they have the capability in altering the activity of a molecule .
Isotypes are found in all normal individuals in the species. The prefix "Iso" means "same in all members of the species."
Some individuals with immunodeficiencies may lack one or more isotypes, (such as IgA deficiency) but normal individuals have all isotypes.
There are 5 classes (Isotypes) of Immunoglobulin, (antibody) based on their structure and biological activity.
These are, IgM, IgG, IgA, IgD and IgE, plus 4 subtypes of IgG (IgG1-4), and 2 of IgA (IgA1, IgA2).
Different types Play different Roles in Immune defence.
IgG, (immunoglobulin G), works efficiently to coat microbes, speeding their uptake by other cells in the immune system. IgG is the most abundant class of antibodies found in blood serum and lymph and active against bacteria, fungi, viruses, and foreign particles. Immunoglobulin G antibodies trigger action of the Complement System.
IgM, is very effective at killing bacteria.
IgA, concentrates in body fluids, such as tears, saliva, the secretions of the respiratory, and the digestive tracts, and guarding the entrances to the body.
IgE, whose natural job probably is to protect against parasitic infections, is the villain responsible for the symptoms of allergy.
IgD remains attached to B cells and plays a key role in initiating early B-cell response.
Apart from having a name, these antibodies also have specific symbols.
Heavy Chain
α - IgA1+2
δ - IgD
γ - IgG1, 2, 3, 4
ε - IgE
μ - IgM
Light Chain
κ
λ
Idiotypes
The term idiotype is sometimes used to describe the collection of multiple idiotopes, and therefore the overall antigen binding capacity, possessed by an antibody.
Idiotypes are unique antigenic determinants (Epitopes), present on individual antibody molecules or on molecules of identical specificity. Identical specificity means that all antibodies molecules have the exact same hypervariable regions. If an antibody binds to an antigen's epitope, the Paratope (the site in the variable region of an antibody or T cell receptor, that binds to an epitope of an antigen.) could become the epitope for another antibody that will then bind to it.
Idiotypes, are the antigenic determinants created by the hypervariable regions of an antibody, and the anti-idiotypic antibodies are those directed against the hypervariable regions of an antibody.
Idiotypes are located on the Fab fragment of the Ig molecules, (which is the antigen binding portion.) Specifically, they are localized at or near the HyperVariable Regions of the heavy and light chains. (HVR's)
In many instances the actual antigenic determinant (idiotype) may include some of the framework residues near the hypervariable region. Idiotypes are usually determinants created by both heavy and light chain HVR's although sometimes isolated heavy and light chains will express the idiotype.
1)Variable region marker - Idiotypes, are a useful marker for a particular variable region.
2) Regulation of immune responses - there is evidence that immune responses may be regulated by anti-Idiotype antibodies directed against our own Idiotypes.
3)Vaccines - In some cases anti-idiotypic antibodies actually stimulate B cells to make antibody and thus they can be used as a vaccine. This approach is being tried to immunize against highly dangerous pathogens that cannot be safely used as a vaccine.
4) Treatment of B cell tumors - Anti-idiotypic antibodies directed against anti-idiotypes on malignant B cells can be used to kill the cells. Killing occurs because of complement fixation or because toxic molecules are attached to the antibodies.
Evidence has been presented that autoimmunity can arise as a result of a cross-reaction between the idiotype on an antiviral antibody, and a host (self) cell receptor for the virus in question. In this case, the host(self) -cell receptor is envisioned as an internal image of the virus, and the anti-idiotype antibodies can react with the host (self) cells.
Immunoglobulin class switching, can be used to change the class of the heavy chain, but not of the light chain.
Allotypes
The prefix "Allo" means "different" in individuals of a species.
Allotypes are hereditary allotypic markers associated with the heavy chains of IgG.
In man the allotypic differences are localized to the constant region of the heavy and light chains.
They are genetically determined variations in plasma proteins which may be recognized as antigenic by other members of the same species.
In other words they are genetic hereditary proteins specified by allelic forms of the Ig genes, which would be seen as antigens in another person.
They are commonly applied to subclasses of immunoglobulins. Allotypes represent slight differences in the amino acid sequences of heavy or light chains of different individuals. Even a single amino acid difference can give rise to an allotypic determinant, although in many cases there are several amino acid substitutions that have occurred.
Individual allotypes are found in individual members of a species.
All allotypes are not found in all members of the species.
Human Ig allotypes are named on the basis of the heavy or light chain on which it is located. Thus, an allotype on a Gamma 1 heavy chain is given the name: G1m(3). An allotype on a Kappa light chain is given the name: Km(1).
Nomenclature, is a system of names used in science:
