Antibody – Structure, Types, Functions and Properties

Antibodies are not found in a specific location, but anytime our immune system encounters antigen or a disease, B cells are activated and antibodies are quickly released into the bloodstream. These immunoglobulins undergo mitosis, resulting in cell division, and manufacture antibodies continually as a result of the production of additional cells. These antibodies linger in the blood for a period of time, but B cells remember these antigens and do the same action whenever they resurface in the body.

What is Antibody?

• Immunoglobulin is also known as an antibody (Ab) (Ig). These are big, Y-shaped blood proteins that plasma cells make. They attach to and penetrate foreign particles. These particles are foreign substances that Antibody attacks.
• Antigens are foreign pathogens that penetrate the body and are capable of eliciting an immune response either in conjunction with a larger molecule or independently after binding with antibodies for a specific immune response. Therefore, antigens trigger the immune system to produce antibodies.
• Antibodies are serum- and tissue-produced globulin proteins (immunoglobulins) that respond specifically with the antigen that prompted their synthesis.
• The blood contains three types of globulins: alpha, beta, and gamma. Gamma globulins are the antibodies.
• Antibodies are one of the key plasma proteins and are often referred to as the “first line of defence” against infection.
• The most important function of antibodies is to protect the body from microbial infections.
• In 1890, Von Behring and Kitasato conducted the first tests demonstrating the physical presence of antibodies. Serum derived from rabbits inoculated with tetanus or diphtheria toxins was shown to prevent illness in mice infected with these infections.
• Tizzoni and Cattani coined the term “antitoxin” in 1880 to refer to the unidentified chemical found in serum that afforded transfer protection.
• Subsequently, Paul Ehrlich and Jules Bordet established through experimentation that a protective response could be induced even against complete cells (erythrocytes).
• The term antitoxin was subsequently superseded by the more encompassing term antibody. In 1939, Tiselius and Kabat made the first successful effort to discover antibody molecules.
• They demonstrated that hyperimmunization increased the serum concentration of -globulins, which carried antibody activity. As -globulins are proteins with a high molecular weight, it was hypothesised that further characterization of antibodies involves breaking them into smaller, more manageable bits.
• Porter digested rabbit immunoglobulin G (IgG) with the proteolytic enzyme papain in 1959.
• These resulted in the formation of two different fragments: Fab (fragment antigen binding), a monovalent fragment with antigen binding activity, and Fc, a fragment that retained the antibody’s effector activities and crystallised easily (fragment crystallizable).
• Using a similar technique, Edelman and Poulik separated myeloma globulins into two different components, which were later designated as heavy (H) and light (L) chains.
• Immunoglobulin (Ig) was coined in 1964 by the World Health Organization (WHO) to replace the term antibody.
• Immunoglobulin consists of antibody globulins, cryoglobulins, macroglobulins, and aberrant myeloma proteins. Consequently, all antibodies are immunoglobulins, but not all immunoglobulins are antibodies.

How Antibody Confer Protection?

Antibodies provide protection through the following mechanisms:

• They inhibit microorganisms from adhering to the mucosal surfaces of the host.
• They inhibit microbial pathogenicity by neutralising poisons and viruses.
• They promote phagocytosis by opsonizing microorganisms.
• They activate complement, resulting in complement-mediated antimicrobial activity.

Properties of Antibodies

• Antibodies function as a protective agent against organisms and external substances.
• This is a glycoprotein.
• Antibodies exist in serum, plasma, and other bodily fluids.
• Antibodies circulate through the blood, where their effector actions, such as phagocytosis, antibody-dependent cell-mediated cytotoxicity (ADCC), opsonization, etc., remove or neutralise the antigen.
• Antibodies are proteins in nature. These are chemically identical to regular Gamma globulin. Immunoglobulin is the word used to describe these related proteins.
• Immune Serum is a serum with elevated antibody levels following infection or vaccination.
• Immunoglobulins are produced by plasma cells and lymphocytes to a lesser extent.
• These account for 20 to 25 percent of total serum proteins. Every antibody is an Immunoglobulin, however not every Immunoglobulin is an antibody.
• They consist of four polypeptide chains: two identical heavy chains (usually 55 kDa apiece) and two identical light chains (25 kDa each). This imparts immunoglobulin with its most recognisable structural trait, a ‘Y’ or ‘T’-shaped overall form.
• The adjectives “heavy” and “light” refer to the chain’s molecular weight. The molecular weight of heavy chains is between 50,000 and 70,000 Da, whereas the molecular weight of light chains is 25,000 Da. The heavier chains are longer than the lighter ones.

Biosynthesis of Immunoglobulins 

• B lymphocytes and plasma cells contribute to immunoglobulin production. Resting B cells produce only minute quantities of immunoglobulins, which are predominantly absorbed into cell membranes.
• Plasma cells, the B cells with the highest degree of differentiation, are specialised to manufacture and secrete vast quantities of immunoglobulins.
• Endoplasmic reticulum is incredibly plentiful in the plasma cells’ vast cytoplasm, which reflects their potential for synthesis.
• Typically, the plasma cell’s heavy and light chains are generated in different polyribosomes.
• Typically, the proportions of heavy and light chains formed on the polyribosomes are balanced, and as a result, both types of chains are mixed to produce Ig molecules with no excess of any chain.
• Either by associating one heavy and one light chain to produce a H–L hemimolecule and then connecting two H–L hemi-molecules to make a single full molecule (H2L2), or by creating H2 and L2 dimers that subsequently associate to form the entire molecule.
• While plasma cells can efficiently produce free light chains, free heavy chains are not often released.
• The heavy chains are generated and transported to the endoplasmic reticulum, where they are glycosylated; nevertheless, the light chains are required for secretion in order to create a full immunoglobulin molecule.
• If light chains are not synthesised or if heavy chains are made in excess, the free heavy chains combine via their CH1 domain with a heavy-chain-binding protein, which is thought to be responsible for their intracytoplasmic retention.
• IgM and IgA are both polymeric antibodies that contain an extra polypeptide chain, known as the J chain.
• All plasma cells generate the J chain, including those that make IgG. However, it is only present in polymeric IgM and IgA forms. The J chain is considered to play a function in starting polymerization.
• Two processes are required to construct IgM proteins. Initial assembly of the monomeric components occurs. Five monomers and one J chain then join through covalent bonds to form a pentameric molecule.

Metabolism of Immunoglobulins

• Immunoglobulin half-life (T 1/2) is one of the most often used measures to assess the catabolic rate of immunoglobulins.
• After equilibrium has been attained, the half-life is the amount of time it takes for the concentration of circulating immunoglobulin to decrease by half.
• This is often determined by injecting a radioisotope-labeled IgG. (131I).
• IgG is the immunoglobulin class with the longest average half-life (21 days), with IgG3 being the exception.
• IgG3 has a significantly shorter half-life (7 days on average) than IgA (5–6 days) and IgM (14 days) (5 days).
• The production rate of IgA1 (24 mg/kg/day) is comparable to that of IgG1 (25 mg/kg/day), although the concentration of IgA1 in the blood is approximately one-third that of IgG1.
• This is because the fractional turnover rate for IgA1 is three times higher (24%/day). IgE has the highest fractional turnover rate and the shortest half-life (74% per day and 2.4 days, respectively).
• IgE has the lowest production rate (0.002 mg/kg/day) compared to IgG (20–60 mg/kg/day).

Structure of Immunoglobulins/Antibody 

An antibody is consist of the following components such as;

1. Two Heavy chains

• A molecule of immunoglobulin contains two heavy chains.
• Each heavy chain has between 420 and 440 amino acids.
• Between one and five disulfide (S—S) bonds hold the two heavy chains together.
• To create a heterodimer, each heavy chain is connected to a light chain by disulfide bonds and noncovalent interactions, such as salt linkages, hydrogen connections, and hydrophobic links (H–L).
• Identical noncovalent interactions and disulfide bridges connect the two identical heavy and light (H–L) chains to produce the basic four-chain (H–L)2 antibody structure.
• The heavy chains of an antibody molecule determine the antibody’s class. For example, IgM includes mu (μ), IgG contains gamma (γ), IgA contains alpha (α), IgD contains delta (δ), and IgE has epsilon (ε) heavy chains.
• Each immunoglobulin class has structurally and antigenically different heavy chains. They vary in size, carbohydrate content, and antigen status.

Classes of immunoglobulins and their heavy chains and subclasses

Class	Heavy chains	Subclasses
IgG	Gamma	γ1, γ2, γ3, γ4,
IgM	Mu	None
IgA	Alpha	α1, α2
IgE	Epsilon	None
IgD	Delta	None

2. Two Light chains

• A molecule of immunoglobulin contains two light chains.
• Each light chain has between 220 and 240 amino acids.
• The light chain is disulfide-bonded to the heavy chain.
• All kinds of immunoglobulins have physically and chemically identical light chains. There are two varieties: kappa (κ) and lambda (λ).
• The amino acids present in their constant regions differ between these two varieties.
• Each immunoglobulin consists of either two or two chains, never both.
• In human serum, the κ and λ chains are present in a ratio of 2 to 1.

3. Variable and constant regions

• Each polypeptide chain of an IgG molecule includes an amino and carboxy terminus.
• The amino terminal portion is referred to as the variable region (V region), while the carboxy terminal portion is referred to as the constant area (C region).
• Both heavy and light chains have parts that are changing and regions that are constant. These regions are formed of domains, which are three-dimensional folded structures with repeating segments.
• Each heavy chain contains one variable domain (VH) and three constant domains (CH).
• IgG and IgA include three CH domains (CH1, CH2, and CH3), while IgM and IgE contain four (CH1, CH2, CH3, and CH4).
• Each light chain has one variable (VL) domain and one constant domain (CL).

a. Variable region

• The amino-terminal 100–110 amino acids of the light or heavy chain are referred to as variable or V regions (VL in light chains and VH in heavy chains).
• V region varies between immunoglobulin classes.
• Three hypervariable areas comprise the variable regions of both light and heavy chains’ variable regions.
• The 5–10 amino acid antigen combining sites Fab of the antibody molecule are found in the hypervariable regions of both the light and heavy chains.
• These antigen-binding sites facilitate the precise association of antibodies with antigens.
• Due mostly to the presence of these hypervariable areas, antibodies possess a high degree of specificity.

b. Constant region

• The carboxyl-terminal portion of a molecule is referred to as the constant (C) region. It consists of two sequences of amino acids.
• The Fc fragment, which crystallises at low ionic circumstances, is located in the heavy chain’s constant area.
• The constant area of the heavy chain has multiple biological purposes. It is responsible for complement activation, binding to cell surface receptors, placental transfer, and numerous other biological processes.
• The light chain’s constant section has no biological purpose.
• A single antibody molecule is composed of two identical heavy chains and two identical light chains, H2L2, or multiples of this basic four-chain structure (H2L2)n.
• There are subisotypes for and chains, leading to the creation of subclasses for each immunoglobulin.

Immunoglobulin Antigen Determinants

Isotypes, allotypes, and idiotypes are the three primary types of immunoglobulin antigen determinants.

1. Isotypes

• The isotype of an immunoglobulin is the unique constant area of the immunoglobulin’s light or heavy chain.
• Immunoglobulins are categorised according to their heavy chain isotypes. In the immunoglobulins IgM, IgG, IgA, IgD, and IgE, heavy chains are identified by the presence of heavy chain markers, such as μ, γ, α, δ and.
• Also distinguishing the light chains are isotype markers, such as κ and λ. Isotypes exist in all individuals of a species.

2. Allotypes

• Allelic variations in both the variable and constant regions of immunoglobulin comprise the allotype.
• On the consistent sections of light and heavy chains, allotype markers are present.
• They are strung on Am on α heavy chains, Gm chains, and Km on κ light chains. Allotype markers are lacking from the μ, δ, and ε  heavy chains, as well as the λ light chain.
• On IgA, over 25 Gm types, 3 Km allotypes, and 2 Am have been identified. Allotypes are inherited in a straightforward Mendelian manner and are present in some but not all members of a species.

3. Idiotypes

• The idiotype denotes a particularity connected with the variable area.
• On the immunoglobulin’s hypervariable region are the idiotype markers.
• Idiotypes are unique to every antibody molecule.
• Antibodies generated against Fab fragments impede the interaction between antigen and antibody.

Antibody Formats

Chimeric antibody

• Chimeric antibodies can be produced through very simple genetic engineering by fusing the immunoglobulin (Ig) variable portions of a selected mouse hybridoma with human Ig constant regions. They can be used as is or as a starting point for further humanization.

Bispecific antibody

• The bispecific antibody can override an effector cell’s specificity for its native target and redirect it to kill a target it would normally ignore. Various cytotoxic cells express distinct activating chemicals (receptors).
• By altering the specificities of target and effector binding domains, diverse effector responses can be directed against the majority of target cell types.
• Alternately, all effector actions (i.e., ADCC, phagocytosis, complement activation, and serum half-life extension) can be delivered by targeting a single binding specificity to serum immunoglobulin.

Recombinant antibody

• Recombinant antibodies are monoclonal antibodies created by the application of recombinant DNA technology.
• Recombinant antibodies are frequently employed in biomedical science and medicine because to their great specificity, sensitivity, and reproducibility.

Role of immunoglobulins in human defense

IgG

• Boosts phagocytosis.
• Neutralizes pathogens and poisons.
• Protects the foetus and infant.

IgM

• Particularly efficient against germs and antigens that agglutinate.
• The initial antibody produced in response to infection.

IgA

• Protection localised on mucosal surfaces.

IgD

• Unknown function of serum.
• Existing on B cells and playing a role in the start of the immune response.

IgE

• Allergic response.
• Possible parasitic worm lysis

Immunoglobulin Classes/Types of Antibody/Antibody Classes

There are five classes of immunoglobulins: (i) immunoglobulin G (IgG), (ii) immunoglobulin M (IgM), (iii) immunoglobulin A (IgA), (iv) immunoglobulin E (IgE), and (v) immunoglobulin D (IgD). Myeloma proteins were initially employed to sequence the amino acids of immunoglobulins. In addition, these proteins were the first immunoglobulins to undergo crystallographic investigations. They offered the first insights of the prototypical immunoglobulin’s domain structure.

1. Immunoglobulin G (IgG)

• IgG antibodies arise later than IgM antibodies in response to an infection, but persist for a longer duration.
• It provides defence against the bacteria present in the blood and tissues. It is equally dispersed in the intravascular and extravascular compartments.
• It is the only immunoglobulin that crosses the placenta; hence, it equips babies with natural passive immunity.
• It participates in precipitation, complement fixation, and toxin and viral neutralisation.
• It binds to bacteria and promotes the process of microbial phagocytosis.

Properties of Immunoglobulin G (IgG)

• With a molecular weight of 150,000 Da, IgG is a 7S immunoglobulin.
• It has the longest half-life among all immunoglobulins, at 23 days.
• IgG is the most common immunoglobulin class in the serum, accounting for approximately 80% of total serum immunoglobulin.
• The most prevalent isotype in plasma, accounting for 80% of the total antibody concentration in serum. It eliminates hazardous chemicals and identifies the antibody-antigen complex.
• It is transported to the placenta via the foetus and serves as protection for the newborn until birth.
• There are four subtypes of IgG: IgG1, IgG2, IgG3, and IgG4. IgG3 and IgG4 are the only ones capable of crossing the placenta.
• IgG heavy chains include two antigen-binding sites and are gamma subclass.
• It facilitates the phagocytosis process and confers immunity to the growing embryo. It protects the body from poisons and pathogens by neutralising them.

Structure of Immunoglobulin G (IgG)

IgG molecules consist of two chains and two or more light chains connected by disulfide links as follows:

• The chain consists of 450 amino acid residues and weighs 51 kDa.
• It is composed of one variable VH domain and a constant (C) area with three CH1, CH2, and CH3 domains.
• Between CH1 and CH2 lies the hinge area.
• Proteolytic enzymes, such as papain and pepsin, cleave the hinge region of an IgG molecule to generate the Fab, F (ab’), and Fc fragments.

Functions of Immunoglobulin G (IgG)

• IgG1, IgG3, and IgG4 are unique in that they are the only immunoglobulins capable of crossing the placental barrier. They perform a crucial function in protecting the developing foetus from infection.
• IgG3, IgG1, and IgG2 are effective at activating the complement in descending order of their efficacy.
• IgG1 and IgG3 bind with great affinity to the Fc receptors on phagocytic cells, hence facilitating opsonization. IgG4 has a moderate affinity for Fc receptors, but IgG2 has a negligible affinity.

Subclasses of Immunoglobulin G (IgG)

In humans, there are four subclasses of IgG corresponding to four γ chain isotypes called γ-1, γ-2, γ-3, and γ-4. IgG1, IgG2, IgG3, and IgG4 have distinct hinge areas and differ in the number and location of disulfide connections connecting two γ chains within each IgG molecule. The amino acid sequence of human γ chain isotypes, excluding the hinge region, differs by only 5%. In the hinge region, cysteine residues that enable the formation of interheavy (γ) chain disulfide linkages are present. IgG1 and IgG4 have two interheavy chain disulfide links, whereas IgG2 and IgG3 have four and eleven, respectively. IgG is equally distributed in the intravascular and extravascular compartments.

a. IgG1 Subclasses

• The most prevalent subclass of IgG antibodies with γ1 heavy chains is IgG1.
• IgG1 is primarily stimulated by soluble protein antigens and membrane proteins, but is frequently accompanied by lower levels of other subtypes.
• As the most prevalent subtype, IgG1 deficiency can lead to a reduction in total IgG levels.

b. IgG2 Subclasses

• IgG2 comprises γ2 heavy chains and is the second most abundant IgG in human serum.
• The response to bacterial capsular polysaccharide antigens is virtually exclusively mediated by IgG2.
• IgG2 antibodies are the only IgG subclass that cannot pass the placenta during pregnancy.
• IgG2 deficiency might result in a weakened immune response to harmful bacteria.

c. IgG3 Subclasses

• IgG3 is the third most prevalent IgG with γ3 heavy chains found in human serum.
• These are especially efficient at inducing effector functions. This antibody is highly proinflammatory and has a shorter half-life.
• IgG3 is also the most potent complement activator and has a high affinity for FcR on phagocytic cells, hence facilitating opsonization.

d. IgG4 Subclasses

• IgG4 is the antibody with the lowest concentration in human serum, which contains γ4 subclasses of heavy chains.
• IgG4 is induced by allergens and is produced following repeated or prolonged antigen exposure in a noninfectious environment.
• IgG4 can pass from the mother to the foetus across the placenta. IgG4 deficiency is extremely uncommon, however elevated levels of IgG4 in the serum have been linked to a variety of organ dysfunctions.

Characteristics of Immunoglobulin G (IgG)

Characteristics	IgG
Structure	Monomer
Percentage of total serum	80%
Location	Blood, lymph, and intestine
Sedimentation coefficient	7
Molecular weight (kDa)	150
Carbohydrate (%)	3
Serum concentration (mg/mL)	12
Half-life (days)	23
Heavy chain	γ1, γ2, γ3, γ4
Light chain	κ or δ
Complement binding	Classical pathway
Placental transport	Positive
Present in milk	Positive
Seromucous secretion	Negative
Heat stability (56oC)	Positive
Binding to tissue	Heterologous

Key Point of IgG

• The most prevalent of all Ig classes.
• The longest half-life among all Ig classes.
• Via the FcRp or Brambell receptor, antigen-presenting cells are able to recycle antigens.
• Agglutinates particle antigens.
• Precipitates soluble antigens.
• This is the only family of antibodies that can pass the placenta FcRn receptor.
• During the first two weeks of life, intestinal cells of neonates also express FcRn.
• Phagocytic cells have FcR; hence, antigens coated with IgG are more readily ingested by these cells.
• Again, by virtue of FcR on Natural Killer cells, these NK cells eliminate antibody-coated invading cells.
• Complement can be activated by IgG attached to cells or IgG aggregated into antigen-antibody complexes.
• Toxin is well neutralised by IgG.
• IgG can inhibit virulence characteristics such as bacterial movement and tissue adhesion.
• IgG can inhibit the capacity of viruses to adhere to target cells.
• IgG is the predominant secondary immune response antibody.

2. Immunoglobulin M (IgM)

• Due to its high valency, pentameric IgM binds antigens with several repeating epitopes, such as virus particles and red blood cells, more efficiently than other isotypes.
• It is more effective at activating complement than IgG. The pentameric structure of a single IgM molecule satisfies the criterion that two Fc regions must be in close proximity for complement activation.
• IgM is the initial immunoglobulin made in response to an antigen. Immunoglobulin provides protection against microbial pathogen invasion of the blood. IgM antibody deficiency is connected with septicemia.
• IgM antibodies have a shorter half-life than IgG antibodies and dissipate sooner. Therefore, the presence of IgM antibodies in serum indicates a recent infection.
• In addition, it is the first immunoglobulin produced by a neonate at approximately 20 weeks of age. IgM is not transferred through the placenta; hence, its presence in a foetus or infant implies intrauterine infection. Therefore, the identification of IgM antibodies in serum is helpful for diagnosing congenital illnesses such as syphilis, rubella, toxoplasmosis, etc.

Properties of Immunoglobulin M (IgM)

• IgM comprises between 5 and 8 percent of total serum immunoglobulins.
• It is primarily dispersed intravascularly.
• It is a molecule (19S) whose molecular weight ranges between 900,000 and 1,000,000 Da (millionaire molecule).
• It has a five-day half-life.
• IgM is the initial antibody generated by B cells in response to a microbial infection.
• It is the biggest antibody and exists as a pentamer.
• It participates in both agglutination and opsonization.
• It features a significant number of antigenic sites on its surface, hence facilitating efficient immune activation.

Structure of Immunoglobulin M (IgM)

• IgM consists of five immunoglobulin subunits (monomeric subunits, IgMs) and one molecule of J chain.
• Each monomeric IgM consists of two light chains ( κ or γ ) and two heavy chains (μ).
• The heavy chains are approximately 20,000 Da bigger than those of IgG, which corresponds to an additional domain in the constant region CH4).
• Two subclasses of IgM (IgM1 and IgM2) with distinct μ chains are described. IgM1 is composed of μ1 chains and IgM2 is composed of μ2 chains.
• The immunoglobulin chain is a 72-kilodalton, 570-amino acid heavy polypeptide chain with one variable region, named VH, and four constant domains, labelled CH1, CH2, CH3, and CH4.
• The μ chain is devoid of a hinge area. A “tail piece” is found at the chain’s carboxyl terminal end. It includes 18-amino acid residues.
• A disulfide bond is formed between the chain and the J chain by a cysteine residue in the carboxy-terminal region of the μ chain.
• Five N-linked oligosaccharides are present in the μ chain of humans.
• B cells express monomeric IgM with a molecular weight of 180,000 Da as a membrane-bound antibody. As discussed previously, the J chain of the IgM molecule was assumed to serve a crucial part in the polymerization process.
• As the antigen-binding molecule in the antigen–antibody complex, IgM is present on the membrane of B cells.
• IgM serum has a higher valency than the other isotypes due to its pentameric shape and ten antigen-binding sites.
• A single IgM molecule may simultaneously bind ten tiny hapten molecules; however, because to steric hindrance, only five or fewer molecules of bigger antigens can be bound.
• IgM antibodies are destroyed by 2-mercaptoethanol without harming IgG antibodies. This is the basis for differential estimation of IgM and IgG antibodies in 2-mercaptoethanol-pretreated serum.

Subclass of Immunoglobulin M (IgM)

There are two subclass of IgM Such as

1. IgM1
2. IgM2

Functions of Immunoglobulin M (IgM)

• The ABO blood group antigens on the surface of RBCs include IgM.
• IgM increases phagocytosis cell consumption.
• It participates in both agglutination and opsonization.
• It features a significant number of antigenic sites on its surface, hence facilitating efficient immune activation.

Key Point of IgM

• The B-cell receptor in mature B-cells is IgM monomer.
• IgM is the first antibody produced by the immune system approximately 7 to 10 days after initial exposure.
• IgM is the first antibody produced during the first five months of life.
• IgM is the most primitive class of antibodies seen in even the most rudimentary animals.
• IgM is the antibody produced against polysaccharide T-independent antigens.
• IgM is a very good agglutinating antibody.
• IgM class isohemagglutinins are found in nature.
• IgM bound to antigen is an excellent complement activator.
• IgM does not pass across the placenta.

Characteristics	IgM
Structure	Pentamer
Percentage of total serum	5-8%
Location	Secretions
Sedimentation coefficient	19
Molecular weight (kDa)	900
Carbohydrate (%)	12
Serum concentration (mg/mL)	1.2
Half-life (days)	5
Heavy chain	μ
Light chain	κ or δ
Complement binding	Classical pathway
Placental transport	Negative
Present in milk	Negative
Seromucous secretion	Negative
Heat stability (56oC)	Positive
Binding to tissue	None

3. Immunoglobulin A (IgA)

Properties of Immunoglobulin A (IgA)

• IgA is the second most abundant immunoglobulin in serum.
• The half-life is between 6 and 8 days.
• Typically present in liquids such as breast milk, serum, saliva, and intestinal fluids. IgA in breast milk protects the gastrointestinal tract of an infant from microbial activity.
• It makes about 13% of the total antibody concentration in the serum and is classified into IgA1 and IgA2 subclasses. IgA1 is the predominant immunoglobulin found in secretions and is also known as the secretory immunoglobulin.
• It is available in both monomeric and dimeric forms.
• It serves as the initial line of defence against infections and suppresses inflammation. In addition, it stimulates the complement system and contributes to the immunological response.

Structure of Immunoglobulin A (IgA)

• IgA molecules contain a α heavy chain that imparts class specificity.
• The  α chain is a 58-kDa polypeptide chain with 470 amino acid residues.
• The chain can be divided into three constant domains labelled CH1, CH2, and CH3 and one variable domain labelled VH. The hinge region is between the CH1 and CH2 domains.
• At the penultimate location of the chain, an extra segment of 18 amino acid residues contains a cysteine residue to which the J chain can be attached via a disulfide bond.

Types of Immunoglobulin A (IgA)

IgA occurs in two forms: serum IgA and secretory IgA.

a. Serum IgA

• This monomeric 7S molecule with a molecular weight of 60,000 Da is present in the serum.
• The half-life is six to eight days.
• It has two subtypes, IgA1 and IgA2, which are the respective α-chain isotypes α-1 and α-2.
• The α-2 chain has two allotypes, A2m (1) and A2m (2), but no disulfide bonds connecting the heavy and light chains.
• Two CH1 locations and five CH3 places distinguish the two chains.
• Thus, humans possess three distinct types of α-heavy chains.

b. Secretory IgA

• It is a dimer or tetramer composed of a J-chain polypeptide and a secretory component, or SC, or secretory piece polypeptide chain.
• The SC is a polypeptide with a molecular weight of 70,000 Da that is generated by mucous membrane epithelial cells.
• It consists of five immunoglobulin-like domains that bind to the IgA dimer’s Fc region domains. A disulfide link between the fifth domain of the SC and one of the IgA dimer chains stabilises this contact.
• Along mucous membrane surfaces, IgA-secreting plasma cells are concentrated. Daily secretory IgA production is larger than that of any other immunoglobulin.
• IgA is the predominant immunoglobulin in external secretions, including breast milk, saliva, tears, and mucus from the bronchial, genitourinary, and digestive tracts.
• IgA stimulates the complement via an alternate pathway as opposed to the traditional mechanism.

Functions of Immunoglobulin A (IgA)

• It defends the mucous membranes from microorganisms that cause disease. It plays a crucial effector function at mucous membrane surfaces, which are the primary entrance points for the majority of pathogens. IgA that is secreted can cross-link big antigens with many epitopes since it is polymeric.
• Attachment of pathogens to mucosal cells is prevented by the binding of secretory IgA to bacterial and viral surface antigens, hence preventing viral infection and bacterial colonisation. Complexes of secretory IgA and antigen are readily captured in mucus and then removed by ciliated epithelial cells of the respiratory tract or by peristalsis of the gastrointestinal tract.
• During the first month of life, breast milk includes IgA and many other chemicals that protect newborns against illness. Because infants’ immune systems are not fully functional, breastfeeding serves a crucial role in preserving their health.
• It has been demonstrated that secretory IgA is an essential line of defence against bacteria (such as Salmonella spp., Vibrio cholerae, and Neisseria gonorrhoeae) and viruses (such as polio, influenza, and reovirus).

Key Point of IgA

• IgA is the antibody that is present in all secretions, including colostrum and milk.
• IgA is especially crucial for respiratory and gastrointestinal defence.
• IgA is essential for the passive acquisition of immunity by a nursing infant.
• IgA enhances the effectiveness of lysozyme, particularly against gram-negative bacteria. IgA neutralises viruses.
• Using the poly-Ig receptor, IgA is transported across membranes.

Characteristics	IgA
Structure	Dimer
Percentage of total serum	10-13%
Location	Blood, lymph, and B cell surface
Sedimentation coefficient	7
Molecular weight (kDa)	160
Carbohydrate (%)	8
Serum concentration (mg/mL)	2
Half-life (days)	6-8
Heavy chain	α1, α2
Light chain	κ or δ
Complement binding	Alternate pathway
Placental transport	Negative
Present in milk	Positive
Seromucous secretion	Positive
Heat stability (56oC)	Positive
Binding to tissue	None

4. Immunoglobulin E (IgE)

Properties of Immunoglobulin E (IgE)

• These are found in the respiratory and gastrointestinal system linings and respond to allergic reactions.
• This is a monomer present in the body that weighs approximately 200,000 Dalton.
• IgE makes up less than one percent of the overall immunoglobulin pool. Serum contains it at an extremely low concentration (0.3 g/mL).
• It is primarily found in the lining of the respiratory and gastrointestinal tracts. IgE is an 8S molecule with a 190,000 Da molecular weight and a 2–3 day half-life.
• IgE, unlike other immunoglobulins, is a heat-labile protein that is easily inactivated at 56°C for 1 hour.

Structure of Immunoglobulin E (IgE)

• IgE molecules consist of two e heavy polypeptide chains and two κ or two λ light polypeptide chains held together by disulfide connections.
• The e chain is a 72 kDa polypeptide chain with 550 amino acid residues. It consists of one variable region, labelled VH, and four constant domains, labelled CH1, CH2, CH3, and CH4.
• This hefty chain is devoid of a hinge area. In humans, the constant portion of the heavy chain contains 428 amino acid residues.
• IgE does not reach the placenta and does not correct the complement.

Functions of Immunoglobulin E (IgE)

• IgE is sometimes referred to as a reaginic antibody, which mediates type I acute hypersensitivity (atopy) reactions.
• IgE is responsible for hay fever, asthma, and anaphylactic shock symptoms. IgE binds to Fc receptors on the membranes of basophils and mast cells in the blood and tissues. Cross-linking of receptor-bound IgE molecules by allergen (antigen) stimulates basophils and mast cells to translocate their granules to the plasma membrane and release their contents into the extracellular environment; this process is referred to as degranulation. As a result, many pharmacologically active mediators are produced, leading to allergy symptoms.
• Localized mast-cell degranulation triggered by IgE may also release mediators that enable the accumulation of certain antiparasitic defense-required cells.

Characteristics	IgE
Structure	Monomer
Percentage of total serum	0.002%
Location	Bound to mast and basophil cell
Sedimentation coefficient	8
Molecular weight (kDa)	190
Carbohydrate (%)	12
Serum concentration (mg/mL)	0.00004
Half-life (days)	1-5
Heavy chain	ε
Light chain	κ or δ
Complement binding	None
Placental transport	–
Present in milk	–
Seromucous secretion	–
Heat stability (56oC)	–
Binding to tissue	Homologous

5. Immunoglobulin D (IgD)

Properties of IgD

• Less than 1% of serum immunoglobulins are IgD.
• It possesses a molecular weight of 180,000 Da and is a 7S monomer.
• IgD has a short half-life of only 2–3 days.
• IgD has a four-chain monomeric structure with two δ heavy chains (each with a molecular weight of 63,000 Da) and two κ or two λ light chains (molecular weight 22,000 Da each).
• It contributes to the synthesis of antibodies by B cells.
• It makes up less than 1% of the total amount of antibodies in serum.
• It functions as a receptor on the surface of B cells and is involved in their activation and differentiation.

Structure of Immunoglobulin D (IgD)

• Immunoglobulin δ chain is a 64-kilodalton, 500-amino acid-residue heavy polypeptide chain composed of one variable region, denoted VH, and three-domain constant regions, denoted CH1, CH2, and CH3.
• In human δ chains, there is also a 58-amino acid residue hinge region.
• This area is encoded by two exons. At its hinge region, IgD is very vulnerable to the activity of proteolytic enzymes.
• Two distinct exons encode the δ chain membrane component.
• The carboxy-terminal region of the human δ chain that is secreted is encoded by a separate exon.
• The δ chain in humans is composed of three N-linked oligosaccharides.

Functions of Immunoglobulin D (IgD)

• Antigen receptor is the most significant function of IgD on B cells. It also affects B cell activity when an antigen is present.
• Blood, mucosal secretions, and the surface of innate immune effector cells also contain trace quantities.

Characteristics	IgD
Structure	Monomer
Percentage of total serum	0.2%
Location	B cell surface, blood, and lymph
Sedimentation coefficient	7
Molecular weight (kDa)	180
Carbohydrate (%)	13
Serum concentration (mg/mL)	0.03
Half-life (days)	2-8
Heavy chain	Δ
Light chain	κ or δ
Complement binding	None
Placental transport	Negative
Present in milk	Negative
Seromucous secretion	Negative
Heat stability (56oC)	Positive
Binding to tissue	None

Abnormal Immunoglobulins

Abnormal immunoglobulins are structurally similar proteins seen in the serum of persons with specific clinical disorders, such as multiple myeloma, heavy chain disease, and cryoglobulinemia, as well as occasionally in healthy individuals.

1. Multiple myeloma

• Bence-Jones (BJ) proteins were the first aberrant proteins discovered in multiple myeloma patients and characterised in 1847.
• These proteins are the light chains of immunoglobulins, and hence exist in either the κ or  λ form. It may manifest in a patient as either κ or  λ, but never as both.
• BJ proteins have the remarkable characteristic of coagulating at 60°C and disintegrating at 80°C. Plasma cells generating IgG, IgA, IgD, or IgE are impacted by multiple myeloma.
• Myeloma including plasma cells that produce IgM is referred to as Waldenstrom’s macroglobulinemia. This syndrome is characterised by an abnormally high production of myeloma proteins (M proteins) and associated light chains (BJ proteins).
• The investigation of myeloma proteins has advanced our understanding of immunoglobulin function.
• In numerous serologic and biochemical studies of the 1950s and 1960s, these “single” or “monoclonal” antibodies derived from the sera of patients with multiple myeloma were utilised.
• They were the primary source of homogenous immunoglobulins until the hybridoma was created in 1974.
• Antibodies were injected into animals to produce antisera, which were then used to explore some of the fundamental features of antibodies. To identify isotypic, allotypic, and idiotypic specificities, for instance, immune sera were absorbed with other myeloma proteins and utilised to identify isotypic, allotypic, and idiotypic specificities.

2. Heavy chain disease

• Heavy chain disease is a distinct ailment that is a lymphoid neoplasia defined by an overproduction of immunoglobulin heavy chains.

3. Cryoglobulinemia

• Cryoglobulinemia is defined by the presence of cryoglobulins in the blood. The condition is frequently encountered in people with macroglobulinemia, systemic lupus erythematosus, and myelomas, although it is not always connected with disease.
• The majority of cryoglobulins are IgG, IgM, or their mixed precipitates. In cryoglobulinemia, the patient’s serum precipitates with cooling and reforms upon warming.

The Immunoglobulin Superfamily

• Earlier descriptions of the architecture of distinct immunoglobulin heavy and light chains indicate that they have a common evolutionary ancestor.
• Specifically, the domain structure of all heavy- and light-chain classes is immunoglobulin-fold.
• The occurrence of this unique structure in all immunoglobulin heavy and light chains shows that their encoding genes originated from a primordial gene encoding a polypeptide of approximately 110 amino acids.
• The multiple heavy- and light-chain genes could have been created via gene duplication and subsequent divergence.
• A large number of membrane proteins have been shown to include one or more immunoglobulin-like domains. Each of these membrane proteins is categorised as an immunoglobulin superfamily member.
• The term superfamily is applied to proteins whose corresponding genes originated from a single ancestral gene encoding the fundamental domain structure.
• These genes have evolved separately and have no genetic connection or shared function. In addition to immunoglobulins, the following proteins are representative members of the immunoglobulin superfamily:
  • The heterodimer Ig-α/Ig-β is a component of the B-cell receptor.
  • Poly-Ig receptor, which contributes to secretory IgA and IgM.
  • T-cell receptor.
  • Comprises T-cell accessory proteins such as CD2, CD4, CD8, CD28, and the δ, γ, ε and chains of CD3.
  • Class I and class II molecules of the major histocompatibility complex.
  • 2-microglobulin is an invariable protein linked with MHC molecules of class I.
  • Multiple cell-adhesion molecules, such as VCAM-1, ICAM-1, ICAM-2, and LFA-3.
  • Protein obtained from platelets.
• Numerous more proteins are members of the immunoglobulin superfamily.
• All members of the immunoglobulin superfamily have not yet been subjected to X-ray crystallographic study.
• Despite this, the fundamental amino acid sequence of these proteins indicates that they all possess the immunoglobulin-fold domain. Specifically, all members of the immunoglobulin superfamily contain at least one or more lengths of approximately 110 amino acids, capable of organisation into pleated sheets of antiparallel strands, typically with an invariant intrachain disulfide bond that closes a 50–70 residue-long loop.
• The majority of immunoglobulin superfamily members cannot bind antigen. Consequently, the Ig-fold structure present in so many membrane proteins must serve a purpose other than antigen binding.
• One theory is that the immunoglobulin structure facilitates membrane protein interactions.
• As previously described, interactions can occur between the faces of pleated sheets of both homologous and nonhomologous immunoglobulin domains (e.g., CH2/CH2 interaction and VH/VL and CH1/CL contacts).

Functions Of Antibodies / Immunoglobulins

The Fc region binds diverse immune cell receptors (such as phagocytes) and performs many effector actions.

Opsonization

• Antibodies (mostly IgG1 and IgG3) can function as opsonins by attaching to the pathogen, hence enhancing phagocyte recognition.
• Phagocytes then commence phagocytosis by binding to the antibodies via their Fc receptors.

Neutralization

• Antibodies can prevent pathogens from entering cells by obstructing certain bacterial or viral cell surface components.
• As a result, some viruses and bacterial toxins are neutralised. To be effective, neutralising antibodies must have a high affinity; IgG and IgA antibodies have the highest efficacy.

Antibody-Dependent Cell-Mediated Cytotoxicity

• Target cells are bound and opsonized by antibodies. The Fc part of the antibody is subsequently recognised by natural killer cells, which release cytotoxic granules (perforin and granzymes) into the target cell to induce apoptosis.
• Additionally, they emit interferons that attract phagocytes.

Complement Activation

• IgM or IgG antibodies can activate the classical complement system when they attach to microbial surfaces.
• This results in the release of C3b, which functions as an opsonin, and other complement components that comprise the membrane attack complex.
• MAC punctures the plasma membrane of the pathogen, resulting in cell lysis and death.

Immune Complexes

• Multiple antigens and antibodies can join together to generate immunological complexes.
• Complex formation restricts the capacity of antigens to diffuse, making it easier for phagocytes to locate and consume infections via phagocytosis.

Functions of Antibody in Brief

The subsequent are some of the primary roles of antibody:

• Attaches to pathogens.
• Activates the immune system in the presence of pathogenic microorganisms.
• Specifically targets viral pathogens.
• Facilitates phagocytosis.
• Antibody provides long-lasting protection against diseases since it survives for years after antigen’s absence.
• It neutralises bacterial toxins and binds the antigen to boost its effectiveness.
• In addition, they serve as the initial line of defence for mucosal surfaces.
• They consume cells through phagocytosis.
• Few antibodies can attach to the antigens found on diseases. This aggregates the pathogen, which persists in the secretions. Antigen is also discharged when the secretion is ejected.

Application of Antibody

• The detection of specific antibodies is a standard way of medical diagnosis, and applications like serology rely on these techniques.
• Targeted monoclonal antibody treatment is used to treat rheumatoid arthritis, multiple sclerosis, psoriasis, and numerous types of cancer, such as non-lymphoma, Hodgkin’s colorectal cancer, head and neck cancer, and breast cancer.
• Antibodies to Rho(D) immune globulin are specific for human RhD antigen. Anti-RhD antibodies are delivered as part of a prenatal therapy protocol to prevent sensitization that may occur when a Rh-negative woman gives birth to a Rh-positive child.

FAQ

References

• Chiu ML, Goulet DR, Teplyakov A, Gilliland GL. Antibody Structure and Function: The Basis for Engineering Therapeutics. Antibodies (Basel). 2019 Dec 3;8(4):55. doi: 10.3390/antib8040055. PMID: 31816964; PMCID: PMC6963682.
• Schroeder HW Jr, Cavacini L. Structure and function of immunoglobulins. J Allergy Clin Immunol. 2010 Feb;125(2 Suppl 2):S41-52. doi: 10.1016/j.jaci.2009.09.046. PMID: 20176268; PMCID: PMC3670108.
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