immunology Immunology is a branch of medicineImmunology for Medical Students, Roderick Nairn, Matthew Helbert, Mosby, 2007 and biology that covers the medical study of immune systems in humans, animals, plants and sapient species. In such we can see ther ...

, affinity maturation is the process by which T_FH cell-activated

B cell B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or ...

s produce antibodies with increased affinity for

antigen In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...

during the course of an immune response. With repeated exposures to the same antigen, a host will produce antibodies of successively greater

affinities In post-classical history, an affinity was a collective name for the group ( retinue) of (usually) men whom a lord gathered around himself in his service; it has been described by one modern historian as "the servants, retainers, and other foll ...

. A secondary response can elicit antibodies with several fold greater affinity than in a primary response. Affinity maturation primarily occurs on membrane immunoglobulin of

germinal center Germinal centers or germinal centres (GCs) are transiently formed structures within B cell zone (follicles) in secondary lymphoid organs – lymph nodes, ileal Peyer's patches, and the spleen – where mature B cells are activated, prolife ...

B cells and as a direct result of

somatic hypermutation Somatic hypermutation (or SHM) is a cellular mechanism by which the immune system adapts to the new foreign elements that confront it (e.g. microbes), as seen during class switching. A major component of the process of affinity maturation, SHM ...

(SHM) and selection by T_FH cells. __TOC__

''In vivo''

The process is thought to involve two interrelated processes, occurring in the germinal centers of the secondary lymphoid organs: #

Somatic hypermutation Somatic hypermutation (or SHM) is a cellular mechanism by which the immune system adapts to the new foreign elements that confront it (e.g. microbes), as seen during class switching. A major component of the process of affinity maturation, SHM ...

: Mutations in the variable, antigen-binding coding sequences (known as

complementarity-determining regions Complementarity-determining regions (CDRs) are part of the variable chains in immunoglobulins (antibodies) and T cell receptors, generated by B-cells and T-cells respectively, where these molecules bind to their specific antigen. A set of CDRs co ...

(CDR)) of the immunoglobulin genes. The mutation rate is up to 1,000,000 times higher than in cell lines outside the lymphoid system. Although the exact mechanism of the SHM is still not known, a major role for the activation-induced (cytidine) deaminase has been discussed. The increased mutation rate results in 1-2 mutations per CDR and, hence, per cell generation. The mutations alter the binding specificity and binding affinities of the resultant antibodies. # Clonal selection: B cells that have undergone SHM must compete for limiting growth resources, including the availability of antigen and paracrine signals from T_FH cells. The

follicular dendritic cells Follicular dendritic cells (FDC) are cells of the immune system found in primary and secondary lymph follicles (lymph nodes) of the B cell areas of the lymphoid tissue. Unlike dendritic cells (DC), FDCs are not derived from the bone-marrow hem ...

(FDCs) of the germinal centers present antigen to the B cells, and the B cell progeny with the highest affinities for antigen, having gained a competitive advantage, are favored for positive selection leading to their survival. Positive selection is based on steady cross-talk between T_FH cells and their cognate antigen presenting GC B cell. Because a limited number of T_FH cells reside in the germinal center, only highly competitive B cells stably conjugate with T_FH cells and thus receive T cell-dependent survival signals. B cell progeny that have undergone SHM, but bind antigen with lower affinity will be out-competed, and be deleted. Over several rounds of selection, the resultant secreted antibodies produced will have effectively increased affinities for antigen.

''In vitro''

Like the natural prototype, the ''in vitro'' affinity maturation is based on the principles of mutation and selection. The ''in vitro'' affinity maturation has successfully been used to optimize antibodies, antibody fragments or other

peptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. ...

molecules like antibody mimetics. Random mutations inside the CDRs are introduced using

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, vi ...

, chemical

mutagen In genetics, a mutagen is a physical or chemical agent that permanently changes genetic material, usually DNA, in an organism and thus increases the frequency of mutations above the natural background level. As many mutations can cause cancer i ...

s or

error-prone PCR In molecular biology, mutagenesis is an important laboratory technique whereby DNA mutations are deliberately engineered to produce libraries of mutant genes, proteins, strains of bacteria, or other genetically modified organisms. The various ...

. In addition, the genetic diversity can be increased by chain shuffling. Two or three rounds of mutation and selection using display methods like

phage display Phage display is a laboratory technique for the study of protein–protein, protein–peptide, and protein– DNA interactions that uses bacteriophages (viruses that infect bacteria) to connect proteins with the genetic information that encodes ...

usually results in antibody fragments with affinities in the low nanomolar range.

References

{{reflist Immunology