Function
Basic Immunoglobulin Structure and Function
Antibodies or immunoglobulins have two light chains and two heavy chains in a light-heavy-heavy-light structure arrangement. The heavy chains differ among classes. They have one Fc region that mediates biological functions (e.g., the binding capacity to cellular receptors) and a Fab region containing antigen-binding sites. The chains are folded into regions called domains. There are 4 or 5 domains in the heavy chain, depending on their class, and two domains in the light chain. The hypervariable regions (HRR) contain the antigen-binding sites. There are three HRRs in the V domains of each light and heavy chain. These fold into regions that produce two antigen-binding sites at the tip of each monomer. All antibodies exhibit one or more functions (bifunctional), including activation of the complement system, opsonization of microbes to be easily phagocytosed, prevention of attachment of the microbes to mucosal surfaces, and neutralization of toxins and viruses.[1]
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Immunoglobulin M
IgM has a molecular weight of 970 Kd and an average serum concentration of 1.5 mg/ml. It is mainly produced in the primary immune response to infectious agents or antigens. It is a pentamer and activates the classical pathway of the complement system. IgM is regarded as a potent agglutinin (e.g., anti-A and anti-B isoagglutinin present in type B and type A blood, respectively), and a monomer of IgM is used as a B cell receptor (BCR).[3]
Immunoglobulin G
IgG is a monomer with an approximate molecular weight of 146 Kd and a serum concentration of 9.0 mg/mL. IgG is said to be divalent, i.e., it has two identical antigen-binding sites that comprise 2 L chains and 2 H chains joined by disulfide bonds. IgG is synthesized mostly in the secondary immune response to pathogens. IgG can activate the classical pathway of the complement system, and it also is highly protective. The four subclasses of IgG include IgG1, IgG2, IgG3, and IgG4. IgG1 is around 65% of the total IgG. IgG2 forms an important host defense against bacteria that are encapsulated. IgG is the only immunoglobulin that crosses the placenta as its Fc portion binds to the receptors present on the surface of the placenta, protecting the neonate from infectious diseases.[4] IgG is thus the most abundant antibody present in newborns.
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Immunoglobulin A
IgA appears in 2 different molecular structures: monomeric (serum) and dimeric structure (secretory). The serum IgA has a molecular weight of 160 Kd and a serum concentration of 3 mg/mL. Secretory IgA (sIgA) has a molecular weight of 385 Kd and a mean serum concentration of 0.05 mg/mL. IgA is the major antibody in secretions found in saliva, tears, colostrum, intestinal, genital tract, and respiratory secretions.
It appears in mucosa membranes as a dimer (with a J chain when secreted) and protects the epithelial surfaces of the digestive, respiratory, and genitourinary systems. IgA possesses a secretory component that prevents its enzymatic digestion. It activates the alternative pathway of activation of the complement system.[5]
Immunoglobulin E
IgE is a monomer. It has a molecular weight of 188 Kd and a serum concentration of 0.00005 mg/mL. It protects against parasites and binds to high-affinity receptors on mast cells and basophils, causing allergic reactions.[6][7][8] IgE is considered the most important host defense against different parasitic infections, including Strongyloides stercoralis, Trichinella spiralis, Ascaris lumbricoides, and hookworms Necator americanus and Ancylostoma duodenale.
Immunoglobulin D
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IgD is a monomer with a molecular weight of 184 Kd. IgD is present in a meager amount in the serum (0.03 mg/mL) and has an unknown function against pathogens. It is regarded as a BCR.[9] IgD may play an essential role in antigen-triggered lymphocyte differentiation.[2]
Receptors for Immunoglobulins
Immunoglobulins should interact with receptors to fulfill various biological functions mainly expressed on mononuclear cells, mast cells, neutrophils, natural killer cells, and eosinophils. Again, binding to these receptors is essential for immunoglobulin functions. It promotes several activities, including phagocytosis of bacteria (opsonization); mast cell degranulation (as seen in type I hypersensitivity or allergic response); killing of tumors; and activation of antigen-presenting cells including macrophages and dendritic cells, which present antigens to T lymphocytes for the generation of cellular and humoral immune responses.[10]
The following are immunoglobulin receptors:
Genetics of Immunoglobulins
The immune system can respond to many antigens by generating various immunoglobulins produced by plasma cells. V and J gene segments encode immunoglobulin light chains. The above genes, in addition to D gene segments, encode the heavy chains. The mechanisms that contribute to this great diversity of immunoglobulin specificities include somatic mutation (immunoglobulin heavy and light chain genes undergo structural modifications after antigen stimulation) and the presence of multiple V-region genes in the germline (antibody diversity also arises when numerous V genes are recombining with J and D segments). Gene conversion, recombinational inaccuracies, nucleotide addition, and assorted heavy and light chains also contribute to the diversity of immunoglobulin molecules.[11][12][13]
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