2.5.9 Antigen-Antibody Complex

Formation of the Antigen-Antibody Complex:

1. Specific Binding:

• Each antibody has two antigen-binding sites located in its variable region.
• The antigen-binding sites are complementary in shape to a specific antigen, ensuring specificity.

2. Lock-and-Key Mechanism:

• The interaction between an antibody and an antigen works like a lock-and-key, where the antigen fits perfectly into the antibody's binding site.

3. Complex Formation:

• Once the antigen and antibody bind, they form a stable antigen-antibody complex, marking the antigen for destruction by the immune system.

Functions of the Antigen-Antibody Complex:

1. Neutralisation:

• Antibodies block the binding sites of pathogens, preventing them from attaching to and infecting host cells.
• Antibodies can also neutralise toxins released by pathogens.

2. Agglutination:

• Antibodies can bind to multiple antigens at once due to their two binding sites, causing pathogens to clump together.
• This makes it easier for phagocytes to engulf the pathogens.

3. Opsonisation:

• The antigen-antibody complex acts as a marker for phagocytes, enhancing the efficiency of phagocytosis.

4. Activation of Complement System:

• Antigen-antibody complexes can trigger the complement cascade, leading to the lysis (bursting) of pathogens.

Steps in the Immune Response Involving the Antigen-Antibody Complex:

1. Antigen Detection:

• Antigens on the surface of a pathogen are recognised by specific antibodies.

2. Binding and Complex Formation:

• Antibodies bind to antigens, forming the antigen-antibody complex.

3. Immune System Activation:

• The complex attracts phagocytes or activates other immune mechanisms like the complement system.

4. Destruction of Pathogens:

• Phagocytes engulf the antigen-antibody complex, breaking it down in lysosomes.

Biological Importance of the Antigen-Antibody Complex:

• Ensures specificity in targeting pathogens.
• Prevents the spread of infections by neutralising pathogens.
• Enhances the efficiency of the immune system by recruiting phagocytes and activating complement proteins.