• 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