Immune system of human beings works in a remarkable fashion. It has its own mechanism of identifying and destroying the 'foreign' substances. And this is where antigens and antibodies come into picture. 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. These include four classes of IgG, two classes of IgA and one class IgM, IgE, and IgD each. Antibodies are present in the body fluids and bloodstream of all vertebrates. These are Y shaped structures with two arms, consisting of four polypeptides. Each arm comprises a light and heavy chain. 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. Action Mechanism 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. One remarkable property of immune system is its ability to produce antibodies against every antigen it encountered. Even if you catch an infection just once in your lifetime, your immune system will remember the antibody for it and produce it, the next time you catch the same infection. Human body can produce and remember around one 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. It is important to identify the type of antigen prior to blood transfusion to minimize the problem of rejection. 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.