2.5.8 Antibodies

Structure of Antibodies:

1. Basic Structure:

• Antibodies are made of four polypeptide chains:
• Two heavy chains (longer).
• Two light chains (shorter).
• These chains are linked by disulphide bonds, forming a Y-shaped molecule.

2. Variable and Constant Regions:

• Variable region:
• Found at the tips of the "Y".
• Contains the antigen-binding sites and is specific to a particular antigen.
• Constant region:
• The rest of the molecule.
• Determines the antibody's class (e.g., IgG, IgA) and interacts with immune cells.

3. Antigen-Binding Sites:

• Each antibody has two binding sites, enabling it to bind to two antigens simultaneously.

Functions of Antibodies:

4. Antigen-Antibody Complex Formation:

• Antibodies bind specifically to antigens, forming antigen-antibody complexes.
• This marks the pathogen for destruction by other immune cells.

5. Agglutination:

• Because antibodies have two binding sites, they can bind to multiple antigens, causing pathogens to clump together.
• This makes it easier for phagocytes to engulf the pathogens.

6. Neutralisation:

• Antibodies block pathogen binding sites, preventing them from infecting host cells.
• They can also neutralise toxins produced by pathogens.

7. Opsonisation:

• Antibodies act as opsonins, tagging pathogens to enhance recognition and destruction by phagocytes.

8. Activation of the Complement System:

• Some antibodies trigger a cascade of reactions known as the complement system, leading to the lysis (bursting) of the pathogen.

Types of Antibodies (Immunoglobulins):

9. $IgG:$

• Most abundant antibody in blood and extracellular fluid.
• Provides long-term immunity.

10. $IgA:$

• Found in mucosal areas (e.g., respiratory and digestive tracts).
• Protects against infection in body secretions like saliva and tears.

11. $IgM:$

• First antibody produced during an immune response.
• Effective in forming antigen-antibody complexes.

12. $IgE:$

• Involved in allergic responses.

13. $IgD:$

• Found on the surface of immature B cells.

Summary of Antibody Action:

Function	Mechanism
Antigen binding	Forms antigen-antibody complexes to mark pathogens for destruction.
Agglutination	Clumps pathogens together for easier phagocytosis.
Neutralisation	Prevents pathogens from binding to host cells or neutralises toxins.
Opsonisation	Tags pathogens to enhance phagocytosis.
Complement activation	Initiates a cascade leading to pathogen lysis.