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What Are Antibodies? – Antibodies Meaning

Antibodies (Ab) are large Y-shaped proteins that are released in response to foreign substances entering our body from the immune system. They are also known as immunoglobulins (Ig). The antibodies function against foreign substances, known as antigens. The shape of the antibody is such that it consists of sites that are made for attachment with the antigens. The antigen-binding sites of the antibody is a diverse structure that can recognise millions of different antigens.

Structure of Antibodies

  • The antibodies are roughly 10nm in size and weigh about 150 kDa.
  • The Y shape of the protein is made up of three globular regions.
  • An antibody is made up of four polypeptide chains: two similar heavy chains and two similar light chains that are held together by disulfide bonds.
  • Each polypeptide chain is made up of about 110 amino acids.
  • The light chain has one constant domain (CL) and one variable domain (VL).
  • Similarly, the heavy chain has three or four constant domains CH1, CH2, CH3, and CH4 and one variable domain (VH).

Classes of Antibodies

The antibodies are divided into five classes based on their locations, biological properties and interaction with antigens in mammals. They are also known as isotypes. The five isotypes are:

Isotype

Function

IgG

It has four subclasses. It is involved in the majority of antibody-antigen interactions. It is the only antibody that is capable of crossing the placenta and providing passive immunity to the foetus.

IgM

It is found in the surface of B cells. It has a pentameric structure and high avidity. It takes part in humoral immunity.

IgE

It is involved in allergic reactions. It is known to bind to allergens to trigger the release of histamine from basophils and mast cells.

IgD

It is an antigenic receptor that is found on the B cells. It activates mast cells and basophils to release antimicrobial factors.

IgA

It is found in mucosal layers such as GI tract, respiratory tract and urogenital tract. It prevents the colonisation of pathogens in such areas.

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