Microbiology and Immunology

Antibody and antigen WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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arm-tip regions are typically those that bind to the antigen.

These portions of the antibody are also known as the antigenic

determinants, or the epitopes.

There are several different types of biochemical interac-

tions between the antibody’s epitopes and the target regions on

the antigen. Hydrogen bonds are important in stabilizing the

antibody-antigen association. In addition, other weak interac-

tions (e.g., van der Waals forces, hydrophobicinteractions,

electrostatic forces) act to tighten the interaction between the

regions on the antibody and the antigen.

The hydrogen bonds that are important in antigen-anti-

body bonding form between amino acids of the antibody and

the antigen. Water molecules that fill in the spaces between the

antibody and the antigen create other hydrogen bonds. The

formation of hydrogen bonds between other regions of the

antibody and antigen, and the water molecules stabilizes the

binding of the immune molecules.

The three-dimensional shape of the molecules is also an

important factor in binding between an antibody and an anti-

gen. Frequently, the antibody molecule forms a pocket that is

the right size and shape to accommodate the target region of

the antigen. This phenomenon was initially described as the

“lock and key” hypothesis.

The exact configuration of the antibody-antigen binding

site is dependent on the particular antigen. Some antigens have

a binding region that is compact. Such a region may be able to

fit into a pocket or groove in the antibody molecule. In con-

trast, other antigen sites may be bulky. In this case, the bind-

ing site may be more open or flatter.

These various three dimensional structures for the bind-

ing site are created by the sequence of amino acids that com-

prise the antibody protein. Some sequences are enriched in

hydrophobic (water-loving) amino acids. Such regions will

tend to form flat sheets, with all the amino acids exposed to

the hydrophilic environment. Other sequences of amino acids

can contain both hydrophilic and hydrophobic (water-hating)

amino acids. The latter will tend to bury themselves away

from water via the formation of a helical shape, with the

hydrophobic region on the inside. The overall shape of an anti-

body and antigen depends upon the number of hydrophilic and

hydrophobic regions and their arrangement within the protein

molecule.

The fact that the interaction between an antibody and an

antigen requires a specific three-dimensional configuration is

exploited in the design of some vaccines. These vaccines con-

sist of an antibody to a region that is present on a so-called

receptor protein. Antigens such as toxin molecules recognize

the receptor region and bind to it. However, if the receptor

region is already occupied by an antibody, then the binding of

the antigen cannot occur, and the deleterious effect associated

with binding of the antigen is averted.

Antibody antigen reactions tend to be irreversible

under normal conditions. This is mainly due to the establish-

ment of the various chemical bonds and interactions between

the molecules. The visible clumping of the antibody-antigen

complex seen in solutions and diagnostic tests such as the

Ochterlony test is an example of the irreversible nature of the

association.

See alsoImmune system; Immunoglobulins and immunoglob-

ulin deficiency syndromes; Laboratory techniques in

immunology; Protein crystallography

AAntibody and antigenNTIBODY AND ANTIGEN

Antibodies, or Y-shaped immunoglobulins, are proteins found

in the blood that help to fight against foreign substances called

antigens. Antigens, which are usually proteins or polysaccha-

rides, stimulate the immune systemto produce antibodies. The

antibodies inactivate the antigen and help to remove it from

the body. While antigens can be the source of infections from

pathogenic bacteriaand viruses, organic molecules detrimen-

tal to the body from internal or environmental sources also act

as antigens. Genetic engineering and the use of various muta-

tional mechanisms allow the construction of a vast array of

antibodies (each with a unique genetic sequence).

Specific genes for antibodies direct the construction of

antigen specific regions of the antibody molecule. Such anti-

gen-specific regions are located at the extremes of the Y-

shaped immunglobulin-molecule.

Once the immune system has created an antibody for an

antigen whose attack it has survived, it continues to produce

antibodies for subsequent attacks from that antigen. This long-

term memory of the immune system provides the basis for the

practice of vaccinationagainst disease. The immune system,

with its production of antibodies, has the ability to recognize,

remember, and destroy well over a million different antigens.

There are several types of simple proteins known as

globulinsin the blood: alpha, beta, and gamma. Antibodies are

gamma globulins produced by B lymphocyteswhen antigens

enter the body. The gamma globulins are referred to as

immunoglobulins. In medical literature they appear in the

abbreviated form as Ig. Each antigen stimulates the production

of a specific antibody (Ig).

Antibodies are all in a Y-shape with differences in the

upper branch of the Y. These structural differences of amino

acids in each of the antibodies enable the individual antibody

to recognize an antigen. An antigen has on its surface a com-

bining site that the antibody recognizes from the combining

sites on the arms of its Y-shaped structure. In response to the

antigen that has called it forth, the antibody wraps its two

combining sites like a “lock” around the “key” of the antigen

combining sites to destroy it.

An antibody’s mode of action varies with different types

of antigens. With its two-armed Y-shaped structure, the anti-

body can attack two antigens at the same time with each arm.

If the antigen is a toxin produced by pathogenic bacteria that

cause an infection like diphtheriaor tetanus, the binding

process of the antibody will nullify the antigen’s toxin. When

an antibody surrounds a virus, such as one that causes

influenza, it prevents it from entering other body cells.

Another mode of action by the antibodies is to call forth the

assistance of a group of immune agents that operate in what is

known as the plasma complementsystem. First, the antibodies

will coat infectious bacteria and then white blood cells will

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