Antigen
The immune system
is the body’s defense against infectious
organisms and other invaders. Through a series of steps called the immune response, the immune
system attacks of organisms and substance
that invade body system and cause disease.
The immune system
protects the body system from possibly harmful substances by recognizing and responding to antigens.
Antigens are substance
(usually proteins) on the surface of cells virus’s fungi or bacteria. Non
living substances such as toxins, chemicals
drugs and foreign particle (such as splinter) can be also antigens. The immune system
recognizes and destroys substance that contains antigens.
An antigen (from
antibody generating)
or immunogen is a substance that prompts the generation of antibodies and can cause immune
responses. In immunology, an antigen is the substance that binds specifically to the respective antibody.
Antigens are usually proteins
or polysaccharides.
Antibodies
Antibodies (also called immunoglobulins)
are specialized proteins
that travel thorough the blood
stream and are found in bodily fluids. They are utilized by the immune system to
identify and defend against
foreign intruders to the body. These foreign
intruders, or antigens, include any
substance or organism that evokes an immune response. Bacteria, viruses, pollen,
and incompatible blood cell types
are examples
of antigens that cause immune responses. Antibodies recognize specific antigens
by identifying certain areas on the surface of the antigen known as antigenic determinants. Once the specific antigenic determinant is recognized, the antibody will
bind to the determinant.. Antibodies protect against
substances prior to cell
infection.
Antibody Structure
An antibody
or immunoglobulin (Ig) is a Y-shaped molecule. It consists of two
short polypeptide chains called light chains and two longer polypeptide
chains called heavy chains. The two light chains and the two
heavy chains are identical to each other. At the ends of both the heavy and light chains,
in the areas that form the arms of the Y-shaped
structure, are regions known as antigen-binding
sites. The antigen-binding site is the area of
the antibody that recognizes the specific antigenic determinant and binds to the antigen.
Since different antibodies
recognize different
antigens, antigen-binding
sites are different for different antibodies. This area of the
molecule is known as the variable region.
The stem of the Y-shaped molecule is formed by
the longer region of the heavy chains. This region is called the constant
region.
Antibody Classes
Five primary classes of antibodies exist
with each class playing
a distinct role in the human immune response.
These classes are identified as IgG, IgM, IgA, IgD and IgE. Immunoglobulin classes differ in the structure
of the heavy chains in each molecule.
Immunoglobulins (Ig)
IgG: These molecules are the most plentiful in circulation. They can
cross blood
vessels and even the placenta to provide protection to a fetus. The heavy chain
type in IgG is a gamma chain.
IgM: Of all of
the immunoglobulins, these are the most massive.
They contain five
Y-shaped sections each with two light chains and two
heavy chains. Each Y-shaped section
is attached to a joining unit called a J chain. IgM molecules play a major role in the primary
immune response as the initial respondents to new
antigens in the body. The heavy chain
type in IgM is a mu chain.
IgA: Located mainly in body fluids such as sweat, saliva, and mucus, these antibodies prevent antigens from infecting cells and entering the circulatory
system. The heavy
chain type in IgA
is an alpha chain.
IgD: The
role of these antibodies in the immune response is currently unknown. IgD
molecules are located on the surface membranes of mature B cells. The heavy chain type in IgD is a delta chain.
IgE: Found mostly in saliva and mucus, these antibodies
are involved in allergic responses to antigens. The heavy chain type in IgE is an epsilon chain.
There are also a few subclasses
of immunoglobulins in humans. The differences in subclasses are based
on small variations in the heavy chain units of antibodies
in the same class. The light chains found in immunoglobulins
exist in two major forms identified as kappa
and lambda chains.
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