Antibody: structure; classess and functions

 Antibodies, also known as immunoglobulins (Ig), are Y-shaped proteins produced by the immune system in response to the presence of foreign substances, such as pathogens or antigens.

Structure:

• Antibodies consist of four polypeptide chains—two heavy chains and two light chains—arranged in a Y-shaped structure.
• The Y-shaped region contains two identical antigen-binding sites at the tips of the arms.
• The constant region determines the antibody class and interacts with various immune system components.

Classes of Antibodies (Immunoglobulins):

1. IgM (Immunoglobulin M):

   • Pentamer structure (five units).
   • Found in blood and lymph fluid.
   • Plays a role in the early stages of an immune response.

2. IgG (Immunoglobulin G):

   • Most abundant antibody in blood and tissues.
   • Provides long-term immunity.
   • Crosses the placenta, conferring passive immunity to the fetus.

3. IgA (Immunoglobulin A):

   • Found in body fluids like saliva, tears, and breast milk.
   • Provides localized defense on mucosal surfaces.

4. IgD (Immunoglobulin D):

   • Found on the surface of B cells.
   • Functions in the activation of B cells.

5. IgE (Immunoglobulin E):

   • Involved in allergic reactions and defense against parasites.
   • Found on the surface of mast cells and basophils.

Functions of Antibodies:

1. Neutralization:

   • Antibodies can bind to pathogens, preventing them from entering or damaging host cells.

2. Opsonization:

   • Antibodies enhance the phagocytosis of pathogens by marking them for destruction by immune cells.

3. Activation of Complement System:

   • Antibodies can trigger the complement system, promoting the destruction of pathogens.

4. Agglutination:

   • Antibodies can clump together pathogens, making it easier for phagocytes to engulf them.

5. Allergic Response:

   • IgE antibodies play a role in triggering allergic reactions by binding to allergens.

Understanding the structure and functions of antibodies is crucial in comprehending how the immune system defends the body against various infections and foreign invaders.