Structure of Antibodies and Antigens
Antibodies belong to a protein family with large molecules, named immunoglobulins. They are secreted by B-lymphocytes. These immunoglobulins are classified into nine chemically distinct classes.
The region at the tip of these two arms varies as per the type of antibody. Hence, it is called variable region. This region forms the binding site whenever an antibody encounters an antigen of corresponding type.
Antigen on the other hand is a foreign substance that triggers the antibodies. Antigens could be bacteria, virus, pollen or even the cells of other human beings (except an identical twin). Antigens may be even a single molecule with a part of bacteria or virus. They are mostly proteins or polysaccharides.
Antibodies counter antigens in two ways, direct and indirect. In direct mechanism, antigens and antibodies form a lock key arrangement. The antigen and antibody fit in perfectly after which the binding part of the antibody marks the antigen as 'foreign', so that phagocytes can kill it.
In the indirect mechanism, antibodies activate the complement proteins, which renders the antigen ineffective, either by weakening it or clumping its cells. Antibodies provide protection from various antigens at different levels.
A remarkable property of immune system is its ability to produce antibodies against every antigen encountered. Even if you catch an infection once in your lifetime, your immune system will remember the antibody for it and produce it the next time you catch the same infection. Our body can produce and remember around 1 million different types of antibodies.
The most distinguished immune system function is its ability to recognize its own cells. The cells of the immune system have markers in the form that says 'self'. This enables the different types of antibodies to co-exist together. An attack is launched only when the antibody encounters a body that has a marker 'foreign'.
Blood antigens and antibodies form the basis of blood group system and blood transfusion. Blood types are classified according to the presence or absence of a particular antigen in the blood.
Thus in short, antibodies are the soldiers of the body whereas antigen are the enemy cells. Understanding the type of antibodies and antigens becomes imperative while diagnosing immune system disorders.