what are antigens and antibodies

Antigens And Antibodies

what are Antigens and Antibodies? Difference between Antigens and Antibodies? what are the reaction occurs between between Antigens and Antibodies?

Antigen :-

The substance that can produce a specific immune response when it is introduced into the tissue is known as antigen.
The basic characters of an antigen includes:-

  • Antigenisity
  • Immunogenisity

Antigenisity :-

The capability of an antigen is retained it’s antigenic nature after entering into the host is called antigenicity.


The ability of an antigen to stimulate the production of antibody is called immunogenisity.

Physical features of an antigen:-


The degree of foreigness that increases the antigenisity of an antigen.


The part of an antigen that can bind to the antibody is known as epitope.
Antigen should processes molecular weight around 10,000 daltons.

Chemical nature of antigen:-

The majority of antigens are proteins because they have largest array of amino acids.
Blood group antigens are strong immunogents.
Nucleoproteins are strongest antigens because nucleic acid is coupled to protein.
Lipids are non-immunogenic but cardiolipin acts as “hapten”.

Cross reactive antigen:-

Antibody released against specific antigen reacts with another antigen due to the presence of same antigenic determinant { epitope}.
The antigens which are involved in the cross reactions are known as cross reactive agents and the antibodies involved in cross reactions are known as cross reactive antibodies.

 Structure of antibody:-

Antibodies are proteins produced against antigens. Antibodies are protein in nature. Antibodies also known as immunoglobulins because there are globular proteins with immune function.

Rodney porter proposed the basic structure of immunoglobulin. It is t shaped {or} y shaped. Immunoglobulins are made up of 4 polypeptide chains of these two are short they are called light chains and the other to change how longer they are called heavy chains. Which light chain is made up of 214 amino acids having molecular weight of 25,000 dalton’s. Heavy chain is made up of 450 to 700 amino acids having molecular weight of 50,000 dalton’s.
The light chains are of two types namely kappa and lambda. The heavy chains are of five types. They are IgG, IgA, IgM, IgD, and IgE

Structure of antibody
Structure of antibody

One end of each chain is called amino terminal {or} n-terminal end and the other end is called carboxyl terminal end. The four chains are interconnected by interchain disulphide bond. The heavy chain and heavy chain is connected by 1-13 interchain disulphide bonds.
The immunoglobulins consists of two regions namely variable region. Is present at c terminal end. In the variable region the amino acids are variably changes where as at constant region the amino acids are constant. The variable region is formed with both light chain and heavy chain half of the constant region is formed only with their heavy chain.

Based on the functional aspect, two fragments can be recognized in the immunoglobulin. They are fab fragment {or} antigen binding fragment and fc fragment {or} crystallizable fragment. The fab fragment is located at the extermity of two limbs of y shaped ig. It is made up of one complete light chain and amino terminal half of heavy chain. The fab fragment binds with the antigen. Fc fragment is located at the start of y shaped ig. Fc fragment is involved in complete fixation.

Each light chain has two domains namely variable light chain domain {or} vl domain and constant light chain {or} cl domain, vl domain is located at the ‘n’ terminal end. The heavy chain has four domains. Variable heavy chain domain {or} vh domain and constant heavy chain domain i.e., ch1, ch2, ch3,but ige has an external domain i.e., ch3. Each light chain has to intra-chain disulphide bonds and heavy chains has 4 intra-chain disulphide bonds.

light chain has intra-chain disulphide bonds and heavy chains has 4 intra-chain disulphide bonds
light chain has intra-chain disulphide bonds and heavy chains has 4 intra-chain disulphide bonds

Each light chain has hyper variable region known as hotspots. Three hotspots are present in light chain, four hotspots are present in heavy chain. All 7 hotspots come into close proximity to form a paratope that can bind to the epitope of an antigen.

Classification of antibodies:-

There are five classes of immunoglobulins based on the presence of heavy chains. They are:-
IgG, IgA, IgM, IgD, and IgE


Antibody is a glycoprotein.
It is t shaped {or} y shaped.
Molecular weight range from 1,50,000 dalton’s to 9,50,000 dalton’s.


The light chain contains two domains:Vl , Cl.
The heavy chain contents of four domains: Vl , Cl, C2, C3, & C4.
Its molecular weight is around 1,50,000 dalton’s.
It’s sedimentation coefficient is 7 and it’s half life is 25 days.

 Biological properties:-

It is the only antibody that crosses the human placenta. It neutralizes viruses and toxins.


It is second abundant immunoglobulin.
It has two sub classes iga­1, and iga­­2,
Iga­1 has both inter and intra chain disulphide bonds.
Iga­­2 has only interchain disulphide bonds

Serum IgA :-

It is a monomer. It contains two lights and two heavy chains, heavy chains are of α- type
Its sedimentation coefficient is 7s and half life is 6-8 days.
Its molecular weight is around 1,60,000 dalton’s.

Secretary IgA:-

It is a dimer. The two molecules are linked by a protein molecule called secretary piece and joining chain.
Its molecular weight is 5,00,000 dalton’s, secretary piece is around 70,000 daltons and j chain is 15,000 dalton’s.

 Biological properties:-

Secretory iga acts as mucosal paint. It gives immunity against tape worm.
Secretary IgA
Secretary IgA


It is the largest immunoglobulin often referred to as macroglobulin.
Its molecular weight is around 9,50,000 dalton’s and its segmentation coefficient is 19s, its half life is 5 days.

 Biological properties:-

As indication of recent infections.


Igd has two light chains either kappa {or} lambda, the two heavy chains are of delta type. Its molecular weight is 1,50,000 dalton’s.
Its sedimentation coefficient is 7s.

 Biological properties:-

It has no bacteriolytic activity.


It is a monomer made up of two light chains and heavy chains.
The light chain is of kappa {or} lambda. Heavy chain is of epsilon type.
The heavy chain has an extra constant region i.e., Cl, C2, C3, & C4,
Its molecular weight is 1,90,000 dalton’s.
It is responsible for allergic reactions. It is involved in hypersensitivity of allergic reactions.
Ige binds on mast cell degranulates then causing hypersensitivity type-1 {or} anaphylaxis.

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