Antibodies vs Immunoglobulins
(Why are Antibodies called as Immunoglobulins?)
All Antibodies are Immunoglobulins but all Immunoglobulins are NOT Antibodies.
Antibodies are the antigen binding proteins found on the B-cell membrane and secreted by the plasma cells of the immune system. Antibodies are commonly called as ‘IMMUNOGLOBULINS’. In the present post we will see, what is the exact difference between an Immunoglobulin and an Antibody, and also why antibodies are called immunoglobulins?
What are Antibodies?
Ø Antibodies are globular proteins (globulins) present in the serum and tissue fluids.
Ø They form one of the major components of the blood plasma proteins.
Ø In the blood, three types of globulin proteins are present and they are named as alpha (α), beta (β) and gamma (γ) globulins.
Ø All antibodies are gamma (γ) globulins.
Ø Antibodies confer protection against microbial pathogens and, they act as the first line defense against infections.
Ø Antibodies are highly specific and they specifically bind to foreign particles called antigens.
Ø The antibodies can protect us from the invading microbes in four different ways:
1. They can prevent the attachment of microbes to the mucosal surface of the host.
2. They reduce the virulence of the pathogen by neutralizing the toxins and viruses.
3. They facilitate phagocytosis by opsonization of microbes.
4. They can activate the complement system.
Antibodies vs Antitoxin
Ø The antibodies were first discovered as ‘Antitoxins’.
Ø The serum obtained from a rabbit immunized with tetanus toxin when injected into mice offered protection against the tetanus toxin.
Ø These unknown components in the serum which offered protection against the tetanus toxin were named as ‘antitoxin’ by Tizzoni and Cattani in 1891.
Ø Later the exact chemical nature of the antitoxin was subsequently discovered and the term antitoxin was gradually replaced by the term Antibody.
Antibodies vs Immunoglobulins
Ø In 1931, Tiselius and Kabat identified and isolated the antibodies from the blood plasma by electrophoretic mobility assay.
Ø They demonstrated that the hyper-immunization increased the concentration of gamma globulin fraction in the blood plasma.
Ø They also found that this globulin fraction of the plasma also possesses the antibody activity.
Ø Since the majority of the antibody types are found in this globulin fraction, the antibodies are also called as Immunoglobulins.
Ø WHO officially coined the term Immunoglobulin (Ig) for antibodies in 1964.
Ø Currently, the term antibody and immunoglobulin are used synonymously.
Ø Apart from the antibody globulins, the immunoglobulin class also contains other globulins such as cryoglobulin, macroglobulin and abnormal myeloma proteins.
Ø This means, all antibodies are immunoglobulins but all immunoglobulins are not antibodies.
Parija, S.C., (2012), Textbook of Microbiology and Immunology, Ed. 2, Elsevier
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