Question

Describe the structure of an antibody molecule and explain how the high level of variability in the antigenic binding sites is produced?

Describe the ways antibodies are used to fight infections.

Answer 1

The answer to your query is as follows:

The structure of an antibody molecule

Antibodies or immunoglobulins are the glycoprotein structures of the immune system that helps in fighting infections. An antibody is made up of four polypeptide chains

• Two heavy chains
• Two light chains

These two chains join together with the help of disulphide bonds to form a "Y" shaped structure of an antibody molecule. These chains are further divided into variable regions and constant regions of the amino acid chains.

The variable region consists of the amino acid sequence that is present in the tips of the "Y" shaped structure and that differs greatly among different antibodies. It is made up of 110-130 amino acids that create its specificity and helps in binding antigens.

When treated with a protease enzyme, the antibody can cleave this region and produce Fab or fragment antigen binding. This Fab includes the variable ends of an antibody.

The constant region of the polypeptide chains helps in determining the process used to destroy antigen.

Based on the structure of the constant region and the immune function, antibodies can be divided into five major classes. These are IgM, IgG, IgA, IgD, and IgE.

The variable region is also subdivided into two sub-regions namely; Hypervariable (HV) and framework (FR) regions.

As the name suggests, hypervariable regions have a very high ratio of the different amino acids that are present in a given position with respect to the most common amino acid found in that position. They are in direct contact of some portion of the antigen's surface, hence they are also referred as complementarity determining regions, or CDRs.

The Framework regions (FR) regions form a beta-sheet structure of the proteins and serves as a scaffold or a specific place to hold the hypervariable region in position to contact antigen.

The high level of variability in the antigenic binding sites

The antigen binding site is made from the amino-terminal ends or the variable domains of light and heavy chains. These two chains fold to form globular variable domains called as VH and VL.

This small region at the tip of the antibody molecule is highly variable and allows millions of antibodies with small different tip structures, or antigen binding sites on it.

The high level of variability in the antigenic binding sites is produced by a region called as the hypervariable region which allows the wide variety of the antibodies to recognize the different antigens.

The large variety of the diverse population of antibodies is formed by the random combinations of some set of gene segments which code for the different paratopes. The paratope is found at the amino terminal end of the antibody monomer that is generated by the variable domains from the heavy and light chains of the antibody molecule.

It is further followed by random mutations in antibody gene area that creates more diversity in the antigen binding site.

The different ways in which antibodies fight infections:

1) Neutrilization- The process in which the neutralizing antibodies block some parts of the bacterial cell surface to make its attack ineffective. These neutralizing antibodies are secreted and released into the blood and mucosa of the human body. They bind to the pathogen and inactivate the foreign substances or toxins released from them.

2) Complement activation process- In this process, antibodies that are latched on a foreign cell activates a complement that attacks the bacterial cells with the help of an activation of the membrane attack complex. It leads to the lysis of the foreign cell and encourages the inflammation by attracting some specific inflammatory cells chemotactically.

3) Opsonization- The process in which antibodies leads to the phagocytosis of foreign substances or the pathogens with the help of the phagocytic cells. The products of the agglutination and precipitation reactions are the attractive targets for this process.

Hope it helps!