2.5.9 Antigen-Antibody Complex

• An antigen and its complementary antibody have complementary molecular shapes
  • This means that their molecular structures fit into each other

• When an antibody collides (randomly) with a foreign cell that possesses non-self antigens with a complementary shape, it binds with one of the antigens
• When this occurs, the two molecules combine to form an antigen-antibody complex

A diagrammatic version of an antigen-antibody complex showing the basic features (in reality, both antigens and antibodies have highly complex 3D structures)

• As seen in the diagram above, antibodies have at least two antigen-binding sites
• This means they can bind to more than one bacterium or virus at the same time
• This cause groups of the same pathogens to become clumped together
  • This process is known as agglutination

• The binding of antibodies to the antigens either neutralises the pathogen or acts like a marker to attract phagocytes to engulf and destroy the pathogens
  • Due to agglutination, phagocytes can often phagocytose many pathogens at the same time, as they are all clumped together

Agglutinated pathogens cannot move easily, so they are easily engulfed and destroyed by phagocytes