Saturday, November 15, 2008

Antigens and Antigen Receptors

Part 2: Antigens and Antigen Receptors

What is an antigen?


An antigen is a substance that stimulates the production of antibodies and can be recognised by a B cell receptor or antibody, or a T cell receptor. As mentioned before, the part of the antigen which comes into contact with the antigen receptor is called the epitope or antigenic determinant.

Antibody - A protein that fights infection.



2 types of Epitopes

A linear or a sequential epitope is an epitope that is recognized by antibodies by its linear sequence of amino acids, or primary structure.

In contrast, most antibodies recognize a conformational epitope that has a specific three-dimensional shape and its protein structure. Conformational epitopes are recognised by antibodies. When the molecule is denatured, the epitope is destroyed due to hanges in its 3-dimensional configuration.

Do B and T cell receptors recognise the same antigen?

B cell receptors (antibodies) can recognise both linear and conformational epitopes. Take note that B cell receptors which recognises conformational epitopes (on native molecules) will not recognise the denatured protein.

T cell receptors recognise short linear peptides presented by MHC molecules.

MHC stands for Major Histocompatibility Complex. It is a complex of genes coding for a large family of cell-surface proteins that bind peptide fragments of foreign proteins and present them to T cells to induce an immune response.


References:

http://www.web-books.com/eLibrary/Medicine/Physiology/Immune/Antigen_Antibody.gif
http://img.photobucket.com/albums/v305/Aegeri/immunodiagram.jpg
http://www.cartage.org.lb/en/themes/sciences/lifescience/GeneralBiology/Immunology/ImmuneSystem/OrgansCells/Cellsandtissues/BT.gif
http://upload.wikimedia.org/wikipedia/commons/thumb/f/f7/B_cell_activation.png/300px-B_cell_activation.png

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