Antigens and antibodies - AS Biology

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Biology Assignment: Antibodies and Antigens
Jade Quarrie
Antigens are molecules that can stimulate an immune response. Antigens are usually large
molecules that have a specific shape. A foreign antigen will be detected by the immune
system and will stimulate the production of antibodies. These antibodies will be specific
to the antigen. As the antigen is specific to the organism, the antibody is specific to the
pathogen. Antigens are usually a protein or glycoprotein in or on the plasma membrane.
Our own antigens recognised by our immune system do not stimulate any response.
Antibodies are molecules produced by the lymphocytes in the immune system. They are
released in response to an infection. Antibodies are large proteins and are also known as
immunoglobulins. They have a specific shape that is complementary to that of a particular
antigen. Therefore antibodies are specific to particular antigens. Our immune system
must manufacture one type of antibody for every antigen that is detected. Antibodies
attach to antigens and render them harmless.
Antibody molecules are Y-shaped and have two distinct regions. The structure of an
antibody molecule includes the following features.
Four polypeptide chains held together by disulfide bridges
A constant region, which is the same in all antibodies. This enables the antibody to
attach to phagocytic cells and helps in the process of phagocytosis.
A variable region, which has a specific shape and differs from one type of antibody
to another. This is the result of its amino acid sequence. It ensures that the
antibody can attach only to the correct antigen. The shape of the variable region is
complementary to the shape of the antigen, and can bind to that antigen.
Hinge regions, which allow a certain degree of flexibility. These allow the
branches of the Y-shaped molecule to move further apart in order to allow
attachment to more than one antigen.

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Biology Assignment: Antibodies and Antigens
Jade Quarrie
Most antibodies work by attaching to the antigens on
a pathogen. The antigen is a molecule on the cell surface
membrane of the pathogen. The pathogen may have another
use for this molecule. For example, it may be a binding site,
which would be used to bind the host cell. If the antibody
blocks this binding site, the pathogen cannot bind to its
host cells. This is called neutralisation.…read more


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