Antibodies

Antibodies are glycoprotein molecules which are produced in response to an antigen, and react specifically with it in an observable manner. Antibody is also known as immunoglobulins.In man 5 major classes of Immunoglobulins are described.They are
(1) Ig G which is the major serum component.
(2) Ig M:It is a macroglobulin appears first after an antigenic stimulus.
(3).Ig A which is present predominantly in secretions
(4) Ig D which is an important cell membrane receptor form. (5) Ig E is an immunoglobulin
that is raised during allergic responses.
Structure of antibodies
The general formula for antibody is (H2 L2)n. The immunoglobulins are made up of 2 heavy chains and 2 light chains. These are held together by covalent bonds. These bonds are interchain disulphide bridges. Each chain is made of a number of loops. These loops are known as domains. Each domain is formed by intrachain disulphide bonds. There are 2 loop sections per L chain and 4 loop sections per H chain. There are two terminals in each chain. One is called C terminus and the other is called N terminus.

Types of heavy chain
There are 5 different types of H chains. Based on the type of H chain the class of antibody is determined
They are: Gamma - Ig G Alpha, Ig A Mu, IgM Epsilon , Ig E Delta and  IgD
Ig M
Ig M is the main immunoglobulin produced early in primary immune response. It is present on the surface of all uncommitted B lymphocytes. IgM is a pentamer and the valence is 10. It is present largely in blood. It is frequently associated with the immune response to Antigenically complex, blood borne infectious agents. Ig M is the most efficient immunoglobulin in agglutination, complement fixation and other antigen antibody reactions. It is a potent activator of classical pathway of complement. This antibody does not cross placenta. It plays an important role in the defense against bacterial and viral diseases.
Ig G
a. lgG contains two heavy chains and two lightchains; heavy chains being of gamma type. Due to its sedimentation coefficient, it is sometimes referred to as 7S lg. IgG is the major antibody; it constitutes about75-80% of total immunoglobulins in circulation. It is the antibody seen in secondary immuno
response.It can pass from vascular compartment to interstitial space. It can cross-placental barrier,
and protects the newborn child from infections.These maternal antibodies are seen in neonatal
circulation up to 2-4 months.Placental crossing of IgG also explains the Rh iso-immunization. This occurs when mother is Rh-negative, and fetus is Rh-positive, and when ABO system antigens are similar to both
mother and fetus. During parturition, fetal RBCs may enter into maternal circulation, leading to formation of anti-Rh antibodies. During next pregnancy, these antibodies, being IgG in class, can enter into fetal
circulation, causing fetal hemolysis, neonatal jaundice, and in severe cases, neonatal death
or miscarriage. Passively injected anti-Rh antibody, when injected within 24 hrs of delivery of first child, will avert the isoimmunization and can protect future pregnancy. Ig G has two identical antigen binding sites and is bivalent. There are four subclasses namely Ig G1,Ig G2, IgG3 and IgG4. Ig G is the predominant antibody in secondary immune response. It plays an important role in defense against bacteria, viruses. It also neutralizes toxins. It crosses placenta and is found in large quantities in newborns.
Ig A
.IgA usually are dimers (total 4 heavy chains and4 light chains). The J chain connects the dimers.They are the secretory antibodies seen inseromucous secretions of gastrointestinal tract, nasopharyngeal tract, urogenital tract, tears, saliva, sweat, etc.The dimers are stabilized against proteolytic
enzymes by the secretory piece. The secretory piece is produced in liver, reaches to the
intestinal mucosal cells, where it combines with IgA dimer to form the secretory IgA which is then
released
IgE
They are cytophilic antibodies. They mediateallergy (Greek, allo = other; ergon = work),
hypersensitivity and anaphylaxis.They have the property to fix on mast cells and basophils. When certain antigens such as penicillin are injected a few times, IgE class antibodies are produced which anchor on mast
cells. When the same chemical is injected next time, the antigens fix on such antibodies, causing mast cell degranulation, and release of histamine and slow reacting substance.This leads to vasodilatation, hypotension and bronchiolar constriction. This is the basis of penicillin anaphylaxis, hay fever caused by fungus, asthma by pollen and urticaria by absorbed food elements. The peak of this reaction will be at about 30 minutes; hence called immediate type hypersensitivity.IgE level in serum is markedly increased in helminthic infections.IgD
IgD has no antibody function. It may act as antigen receptor on cells. In serum it is present in only trace amounts.

• IgM antibody is the one which appears first. 
• IgG antibodies appear later and stay for longer times. 
• IgA antibodies offer first line ofdefense. 
• IgE antibodies are involved in allergic reactions.