What is an Antibody?Immunoglobulins: Classes and Sub classes

What is an Antibody?
A blood protein produced in response to and counteracting a specific
antigen. Antibodies combine chemically with substances which the body
recognizes as alien, such as bacteria, viruses, and foreign substances in
the blood.
Antibodies, also known as immunoglobulins, are Y-shaped proteins that are
produced by the immune system to help stop intruders from harming the
body. When an intruder enters the body, the immune system springs into
action. These invaders, which are called antigens, can be viruses, bacteria, or
other chemicals. When an antigen is found in the body, the immune system
will create antibodies to mark the antigen for the body to destroy.

The membrane-bound form of an antibody may be called a surface
immunoglobulin (sIg) or a membrane immunoglobulin (mIg).
• It is part of the B cell receptor (BCR), which allows a B cell to detect
when a specific antigen is present in the body and triggers B cell
activation .
• The BCR is composed of surface-bound IgD or IgM antibodies and
associated Ig-α and Ig-β heterodimers, which are capable of signal
transduction .
• A typical human B cell will have 50,000 to 100,000 antibodies bound to
its surface. Upon antigen binding, they cluster in large patches, which
can exceed 1 micrometer in diameter, on lipid rafts that isolate the
BCRs from most other cell signaling receptors. These patches may
improve the efficiency of the cellular immune response. In humans, the
cell surface is bare around the B cell receptors for several hundred
nanometers, which further isolates the BCRs from competing
influences.
Forms

Immunoglobulins: Classes and Subclasses
There are five immunoglobulin classes (isotypes) of antibody
molecules found in serum: IgG, IgM, IgA, IgE and IgD. They are
distinguished by the type of heavy chain they contain.
IgG molecules possess heavy chains known as γ-chains.
IgMs have μ-chains.
IgAs have α-chains.
IgEs have ε-chains; and IgDs have δ-chains. The variation in heavy chain
polypeptides allows each immunoglobulin class to function in a different type
of immune response or during a different stage of the body’s defense.
The amino acid sequences that confer these functional differences are located
mainly within the Fc domain.
Antibody classes also differ in their valency, i.e. the number of arms available to
bind antigen. This arises from the ability of certain immunoglobulins to
form multimers through linkage of their Fc domains via a J chain. For example,
IgM is a pentamer of five identical “Y” shaped monomers. Therefore, the
complete IgM protein contains 10 heavy chains, 10 light chains and 10 antigen
binding arms (giving IgM a valency of 10)

Human immunoglobulin A (IgA) is the second most common human
immunoglobulin in serum. It is secreted in milk and is also the most prevalent
lg in secretions (e.g. tears, saliva and mucous).
IgA is resistant to digestion and can activate the complement pathway when
aggregated. It should also be noted both subclasses of IgA (IgA1 and
IgA2) bind fragment crystallization (Fc) receptors .
IgA: Immunoglobulin A Class
Expressed on the surface of mature B cells, human immunoglobulin D
(IgD) works with IgM in B cell development.
IgD is found in very low levels in serum and does not activate the complement
pathway.
IgD: Immunoglobulin D Class

Human immunoglobulin E (IgE) is expressed on the surface of mature B cells. Human
immunoglobulin IgE is the least abundant Ig in the serum and does not activate the
complement pathway.
Fragment crystallization (Fc) receptors for IgE are found on eosinophils and IgE binds Fc
receptors on mast cells and basophils even before interacting with antigen. As a result of
its binding to basophils and mast cells, IgE is involved in allergic reactions. This happens
when allergen is bound to IgE on cells and releases various pharmacological mediators
which cause allergies.
Human IgG (immunoglobulin G) is expressed on the surface of mature B cells and is the
most prevalent Ig in serum and the major Ig in extravascular spaces. IgG1,
IgG2 and IgG3 are complement activators, with IgG3 being the strongest.
Human immunoglobulin subclasses IgG1 and IgG3 tend to strongly bind fragment
crystallization (Fc) receptors, whereas subclasses IgG2 and IgG4 bind weakly. IgG is also the
only human immunoglobulin to pass from mother to fetus to transfer immunity.
It should be noted, the human immunoglobulin subclass IgG2 does cross the placenta, but
it does so weakly.
IgG: Immunoglobulin G Class
IgE: Immunoglobulin E Class

Human IgM (immunoglobulin M) is expressed on the surface of immature and mature B
cells as monomers. IgM is the third most abundant human immunoglobulin.
IgM is also the first human immunoglobulin to be made by a fetus and virgin B cells
which are challenged with antigen. IgM is a strong complement activator and
agglutinator due to its pentameric structure and binds fragment crystallization (Fc)
receptors.
IgM: Immunoglobulin M Class
Each antibody binds to a
specific antigen; an interaction
similar to a lock and key.

An epitope, also known as antigenic determinant, is the part of
an antigen that is recognized by the immune system, specifically
by antibodies, B cells, or T cells. For example, the epitope is the specific piece of
the antigen to which an antibody binds.
The part of an antibody that binds to the epitope is called a paratope.
Although epitopes are usually non-self proteins, sequences derived from the host
that can be recognized (as in the case of autoimmune diseases) are also epitopes.
The epitopes of protein antigens are divided into two categories
conformational epitopes
linear epitopes based on their structure and interaction with the paratope.
A conformational epitope is composed of discontinuous sections of the
antigen's amino acid sequence. These epitopes interact with the paratope based on
the 3-D surface features and shape or tertiary structure of the antigen. The
proportion of epitopes that are conformational is unknown.
By contrast, linear epitopes interact with the paratope based on their primary
structure. A linear epitope is formed by a continuous sequence of amino acids from
the antigen.
Epitope

The antibody's paratope interacts with the antigen's epitope. An antigen
usually contains different epitopes along its surface arranged discontinuously,
and dominant epitopes on a given antigen are called determinants.
• Antibody and antigen interact by spatial complementarity (lock and key).
• The molecular forces involved in the Fab-epitope interaction are weak and
non-specific – for example electrostatic forces, hydrogen
bonds, hydrophobic interactions, and van der Waals forces.
• This means binding between antibody and antigen is reversible, and the
antibody's affinity towards an antigen is relative rather than absolute.
Relatively weak binding also means it is possible for an antibody to cross-
react with different antigens of different relative affinities.
Often, once an antibody and antigen bind, they become an immune complex,
which functions as a unitary object and can act as an antigen in its own right,
being countered by other antibodies. Similarly, haptens are small molecules
that provoke no immune response by themselves, but once they bind to
proteins, the resulting complex or hapten-carrier adduct is antigenic.
Antibody–antigen interactions

Antibodies structure
Each antibody consists of four polypeptides–
two heavy chains and two light chains joined
to form a "Y" shaped molecule.
This variable region, composed of 110-130
amino acids, give the antibody its specificity
for binding antigen.
This variable region, composed of 110-130
amino acids, give the antibody its specificity
for binding antigen.
The variable region is further subdivided into hyper
variable (HV) and framework (FR) regions. Hyper variable
regions have a high ratio of different amino acids in a given
position, relative to the most common amino acid in that
position. Within light and heavy chains, three hyper
variable regions exist – HV 1, 2 and 3. Four FR regions
which have more stable amino acids sequences separate
the HV regions.

The HV regions directly contact a portion of the antigen's surface. For this
reason, HV regions are also sometimes referred to as complementarity
determining regions, or CDRs. The FR regions form a beta-sheet structure
which serves as a scaffold to hold the HV regions in position to contact
antigen.

There are five types of mammalian Ig heavy chain denoted by the Greek
letters: α, δ, ε, γ, and μ.The type of heavy chain present defines the class of
antibody; these chains are found in IgA, IgD, IgE, IgG, and IgM antibodies,
respectively. Distinct heavy chains differ in size and composition; α and γ contain
approximately 450 amino acids, whereas μ and ε have approximately
550 amino acids.
Each heavy chain has two regions.
• The constant region
• The variable region.
 The constant region is identical in all antibodies of the same isotype, but
differs in antibodies of different isotypes. Heavy chains γ, α and δ have a
constant region composed of three tandem (in a line) Ig domains, and a
hinge region for added flexibility; heavy chains μ and ε have a constant
region composed of four immunoglobulin domains.
 The variable region of the heavy chain differs in antibodies produced by
different B cells, but is the same for all antibodies produced by a single B cell
or B cell clone. The variable region of each heavy chain is approximately 110
amino acids long and is composed of a single Ig domain.
Heavy chain

• In mammals there are two types of immunoglobulin light chain, which are
called lambda (λ) and kappa (κ).
• A light chain has two successive domains: one constant domain and one
variable domain.
• The approximate length of a light chain is 211 to 217 amino acids. Each
antibody contains two light chains that are always identical; only one type
of light chain, κ or λ, is present per antibody in mammals.
• Other types of light chains, such as the iota (ι) chain, are found in
other vertebrates like sharks (Chondrichthyes) and bony fishes (Teleostei)
Light chain
Complementarity Determining Regions (CDRs).
To be specific, variable loops of β-strands, there each on the light (VL) and
heavy (VH) chains are responsible for binding to the antigen. These loops
are referred to as the complementarity determining regions (CDRs)

The arms of the Y, for example, contain the sites that can bind to antigens (in
general, identical) and, therefore, recognize specific foreign objects. This
region of the antibody is called the Fab (fragment, antigen-binding) region
It is composed of one constant and one variable domain from each heavy and light
chain of the antibody.
• The paratope is shaped at the amino terminal end of the
antibody monomer by the variable domains from the heavy and light
chains.
• The variable domain is also referred to as the FV region and is the most
important region for binding to antigens. To be specific, variable loops of
β-strands, there each on the light (VL) and heavy (VH) chains are
responsible for binding to the antigen. These loops are referred to as
the complementarity determining regions (CDRs). The structures of
these CDRs have been clustered and classified by Chothia et al. and
more recently by North et al. and Nikoloudis et al. In the framework of
the immune network theory, CDRs are also called idiotypes.
• According to immune network theory, the adaptive immune system is
regulated by interactions between idiotypes.
(Fragment, antigen-binding) region

The base of the Y plays a role in modulating immune cell activity. This
region is called the Fc (Fragment, crystallizable) region, and is composed of
two heavy chains that contribute two or three constant domains
depending on the class of the antibody.
Thus, the Fc region ensures that each antibody generates an appropriate
immune response for a given antigen, by binding to a specific class of Fc
receptors, and other immune molecules, such as complement proteins. By
doing this, it mediates different physiological effects including recognition
of opsonized particles (binding to FcγR), lysis of cells (binding to
complement), and degranulation of mast cells, basophils,
and eosinophils (binding to FcεR).
• The Fab region of the antibody determines antigen specificity while the Fc
region of the antibody determines the antibody's class effect.
Since only the constant domains of the heavy chains make up the Fc region of an
antibody, the classes of heavy chain in antibodies determine their class effects. This
infers different isotypes of antibodies have different class effects due to their
different Fc regions binding and activating different types of receptors. Possible class
effects of antibodies include: Opsonisation, agglutination, haemolysis, complement
activation, mast cell degranulation, and neutralisation (though this class effect may
be mediated by the Fab region rather than the Fc region). It also implies that Fab-
mediated effects are directed at microbes or toxins, whilst Fc mediated effects are
directed at effector cells or effector molecules

• Neutralization, in which neutralizing antibodies block parts of the
surface of a bacterial cell or virion to render its attack ineffective
• Agglutination, in which antibodies "glue together" foreign cells into
clumps that are attractive targets for phagocytosis
• Precipitation, in which antibodies "glue together" serum-soluble
antigens, forcing them to precipitate out of solution in clumps that are
attractive targets for phagocytosis
• Complement activation (fixation), in which antibodies that are latched
onto a foreign cell encourage complement to attack it with
a membrane attack complex, which leads to the following:
• Lysis of the foreign cell
• Encouragement of inflammation by chemotactically attracting
inflammatory cells
Function
The main categories of antibody action include
the following:

Activation of complement
Antibodies that bind to surface antigens (for example, on bacteria) will attract the first
component of the complement cascade with their Fc region and initiate activation of
the "classical" complement system.[31] This results in the killing of bacteria in two
ways.[5] First, the binding of the antibody and complement molecules marks the
microbe for ingestion by phagocytes in a process called opsonization; these phagocytes
are attracted by certain complement molecules generated in the complement cascade.
Second, some complement system components form a membrane attack complex to
assist antibodies to kill the bacterium directly (bacteriolysis).
• Activated B cells differentiate into either antibody-producing cells
called plasma cells that secrete soluble antibody or memory cells that
survive in the body for years afterward in order to allow the immune system
to remember an antigen and respond faster upon future exposures.
• At the prenatal and neonatal stages of life, the presence of antibodies is
provided by passive immunization from the mother. Early endogenous
antibody production varies for different kinds of antibodies, and usually
appear within the first years of life. Since antibodies exist freely in the
bloodstream, they are said to be part of the humoral immune system.
Circulating antibodies are produced by clonal B cells that specifically respond
to only one antigen (an example is a virus capsid protein fragment).

Activation of effector cells
To combat pathogens that replicate outside cells, antibodies bind to pathogens
to link them together, causing them to agglutinate. Since an antibody has at
least two paratopes, it can bind more than one antigen by binding identical
epitopes carried on the surfaces of these antigens.
• By coating the pathogen, antibodies stimulate effector functions against the
pathogen in cells that recognize their Fc region.
• Those cells that recognize coated pathogens have Fc receptors, which, as the
name suggests, interact with the Fc region of IgA, IgG, and IgE antibodies.
• The engagement of a particular antibody with the Fc receptor on a particular
cell triggers an effector function of that cell; phagocytes
will phagocytose, mast cells and neutrophils will degranulate, natural killer
cells will release cytokines and cytotoxic molecules; that will ultimately
result in destruction of the invading microbe.
• The activation of natural killer cells by antibodies initiates a cytotoxic
mechanism known as antibody-dependent cell-mediated cytotoxicity (ADCC)
– this process may explain the efficacy of monoclonal antibodies used
in biological therapies against cancer. The Fc receptors are isotype-specific,
which gives greater flexibility to the immune system, invoking only the
appropriate immune mechanisms for distinct pathogens.

Medical Applications
Detection of particular antibodies is a very common form of
medical diagnostics, and applications such as serology depend on these
methods.
• For example, in biochemical assays for disease diagnosis, a titer of
antibodies directed against Epstein-Barr virus or Lyme disease is
estimated from the blood. If those antibodies are not present, either
the person is not infected or the infection occurred a very long time ago,
and the B cells generating these specific antibodies have naturally
decayed.
• In clinical immunology, levels of individual classes of immunoglobulins
are measured by nephelometry (or turbidimetry) to characterize the
antibody profile of patient.
• Elevations in different classes of immunoglobulins are sometimes useful
in determining the cause of liver damage in patients for whom the
diagnosis is unclear.
• For example, elevated IgA indicates alcoholic cirrhosis, elevated IgM
indicates viral hepatitis and primary biliary cirrhosis, while IgG is
elevated in viral hepatitis, autoimmune hepatitis and cirrhosis.
.

• Autoimmune disorders can often be traced to antibodies that bind
the body's own epitopes; many can be detected through blood
tests. Antibodies directed against red blood cellsurface antigens in
immune mediated hemolytic anemia are detected with
the Coombs test.
• The Coombs test is also used for antibody screening in blood
transfusion preparation and also for antibody screening
in antenatal women.
• Practically, several immunodiagnostic methods based on detection
of complex antigen-antibody are used to diagnose infectious
diseases, for example ELISA, immunofluorescence, Western
blot, immunodiffusion, immunoelectrophoresis, and magnetic
immunoassay.
• Antibodies raised against human chorionic gonadotropin are used
in over the counter pregnancy tests.
• New dioxaborolane chemistry enables radioactive fluoride (18F)
labeling of antibodies, which allows for positron emission
tomography (PET) imaging of cancer

Disease therapy
Targeted monoclonal antibody therapy is employed to treat diseases such
as rheumatoid arthritis, multiple sclerosis, psoriasis , and many forms
of cancer including non-Hodgkin's lymphoma, colorectal cancer, head and
neck cancer and breast cancer.
Some immune deficiencies, such as X-linked
agammaglobulinemia and hypogammaglobulinemia, result in partial or
complete lack of antibodies. These diseases are often treated by inducing a
short term form of immunity called passive immunity.
Passive immunity is achieved through the transfer of ready-made
antibodies in the form of human or animal serum, pooled immunoglobulin
or monoclonal antibodies, into the affected individual.

Prenatal therapy
 Rh factor, also known as Rh D antigen, is an antigen found on red blood cells;
individuals that are Rh-positive (Rh+) have this antigen on their red blood cells and
individuals that are Rh-negative (Rh–) do not.
 During normal childbirth, delivery trauma or complications during pregnancy, blood
from a fetus can enter the mother's system. In the case of an Rh-incompatible
mother and child, consequential blood mixing may sensitize an Rh- mother to the
Rh antigen on the blood cells of the Rh+ child, putting the remainder of
the pregnancy, and any subsequent pregnancies, at risk for hemolytic disease of
the newborn.
 Rho(D) immune globulin antibodies are specific for human RhD antigen.
 Anti-RhD antibodies are administered as part of a prenatal treatment regimen to
prevent sensitization that may occur when a Rh-negative mother has a Rh-positive
fetus.
 Treatment of a mother with Anti-RhD antibodies prior to and immediately after
trauma and delivery destroys Rh antigen in the mother's system from the fetus.
 It is important to note that this occurs before the antigen can stimulate maternal B
cells to "remember" Rh antigen by generating memory B cells. Therefore, her
humoral immune system will not make anti-Rh antibodies, and will not attack the
Rh antigens of the current or subsequent babies.
 Rho(D) Immune Globulin treatment prevents sensitization that can lead to Rh
disease, but does not prevent or treat the underlying disease itself.

Regulations
• Production and testing
Traditionally, most antibodies are produced by hybridoma cell lines through
immortalization of antibody-producing cells by chemically-induced fusion with myeloma
cells. In some cases, additional fusions with other lines have created "triomas" and
"quadromas". The manufacturing process should be appropriately described and
validated. Validation studies should at least include:
 The demonstration that the process is able to produce in good quality (the process
should be validated)
 The efficiency of the antibody purification (all impurities and virus must be eliminated)
 The characterization of purified antibody
(physicochemical characterization, immunological properties, biological activities,
contaminants, ...)
 Determination of the virus clearance studies
• Before clinical trials
 Product safety testing: Sterility (bacteria and fungi), In vitro and in vivo testing for
adventitious viruses, Murine retrovirus testing... Product safety data needed before the
initiation of feasibility trials in serious or immediately life-threatening conditions, it
serves to evaluate dangerous potential of the product.
 Feasibility testing: These are pilot studies whose objectives include, among others,
early characterization of safety and initial proof of concept in a small specific patient
population (in vitro or in vivo testing).

Preclinical studies
• Testing cross-reactivity of antibody: to highlight unwanted interactions
(toxicity) of antibodies with previously characterized tissues. This study
can be performed in vitro (Reactivity of the antibody or immunoconjugate
should be determined with a quick-frozen adult tissues) or in vivo (with
appropriates animal models). More informations about in vitro cross-
reactivity testing.
• Preclinical pharmacology and toxicity testing: Preclinical safety testing of
antibody is designed to identify possible toxicity in humans, to estimate
the likelihood and severity of potential adverse events in humans, and to
identify a safe starting dose and dose escalation, when possible.
• Animal toxicity studies: Acute toxicity testing, Repeat-dose toxicity testing,
Long-term toxicity testing http://www.animalresearch.info/en/drug-
development/safety-testing/
• Pharmacokinetics and pharmacodynamics testing: Use for determinate
clinical dosages, antibody activities (AUC, pharmacodynamics,
biodistribution, ...), evaluation of the potential clinical effects

1) Antibodies (A) and pathogens (B) free roam in the blood.
2) The antibodies bind to pathogens, and can do so in
different formations such as: opsonization (2a),
neutralisation (2b), and agglutination (2c). 3) A phagocyte
(C) approaches the pathogen, and the Fc region (D) of the
antibody binds to one of the Fc receptors (E) of the
phagocyte. 4) Phagocytosis occurs as the pathogen is
ingested.

Structure prediction
The importance of antibodies in health care and the biotechnology industry
demands knowledge of their structures at high resolution.
This information is used for protein engineering, modifying the antigen binding
affinity, and identifying an epitope, of a given antibody.
• X-ray crystallography is one commonly used method for determining antibody
structures. However, crystallizing an antibody is often laborious and time-
consuming.
• Computational approaches provide a cheaper and faster alternative to
crystallography, but their results are more equivocal, since they do not
produce empirical structures. Online web servers such as Web Antibody
Modeling (WAM) and Prediction of Immunoglobulin Structure (PIGS)enables
computational modeling of antibody variable regions. Rosetta Antibody is a
novel antibody FV region structure prediction server, which incorporates
sophisticated techniques to minimize CDR loops and optimize the relative
orientation of the light and heavy chains, as well as homology models that
predict successful docking of antibodies with their unique antigen.
• The ability to describe the antibody through binding affinity to the antigen is
supplemented by information on antibody structure and amino acid sequences
for the purpose of patent claims.

History
The first use of the term "antibody" occurred in a text by Paul Ehrlich.
The term Antikörper (the German word for antibody) appears in the
conclusion of his article "Experimental Studies on Immunity", published in
October 1891, which states that, "if two substances give rise to two
different Antikörper, then they themselves must be different". However, the
term was not accepted immediately and several other terms for antibody
were proposed; these
included Immunkörper, Amboceptor, Zwischenkörper, substance
sensibilisatrice, copula, Desmon, philocytase, fixateur, and Immunisin. The
word antibody has formal analogy to the word antitoxin and a similar
concept to Immunkörper (immune body in English). As such, the original
construction of the word contains a logical flaw; the antitoxin is
something directed against a toxin, while the antibody is a
body directed against something.

• The biochemical properties of antigen-antibody-binding interactions were
examined in more detail in the late 1930s by John Marrack.
• The next major advance was in the 1940s, when Linus Pauling confirmed the lock-
and-key theory proposed by Ehrlich by showing that the interactions between
antibodies and antigens depend more on their shape than their chemical
composition
• In 1948, Astrid Fagreaus discovered that B cells, in the form of plasma cells,
were responsible for generating antibodies.
Edelman went on to discover that antibodies are composed of disulfide bond-
linked heavy and light chains.
Around the same time, antibody-binding (Fab) and antibody tail (Fc) regions of IgG
were characterized by Rodney Porter.Together, these scientists deduced the
structure and complete amino acid sequence of IgG, a feat for which they were
jointly awarded the 1972 Nobel Prize in Physiology or Medicine.
The Fv fragment was prepared and characterized by David Givol.
While most of these early studies focused on IgM and IgG, other immunoglobulin
isotypes were identified in the 1960s: Thomas Tomasi discovered secretory
antibody (IgA); David S. Rowe and John L. Fahey discovered IgD; and Kimishige
Ishizaka and Teruko Ishizaka discovered IgE and showed it was a class of antibodies
involved in allergic reactions.

Antibody mimetic
• Antibody mimetics are organic compounds that, like
antibodies, can specifically bind antigens.
• They are usually artificial peptides or proteins with a
molar mass of about 3 to 20 kDa.
• Nucleic acids and small molecules are sometimes
considered antibody mimetics, but not artificial
antibodies, antibody fragments and fusion proteins are
composed from these.
• Common advantages over antibodies are better
solubility, tissue penetration, stability towards heat and
enzymes, and comparatively low production costs.
Antibody mimetics such as the Affimer and
the DARPin have being developed and commercialised
as research, diagnostic and therapeutic agents.

What is an Antibody?Immunoglobulins: Classes and Sub classes

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What is an Antibody?Immunoglobulins: Classes and Sub classes