Antigens & Antibodies (Cambridge (CIE) O Level Biology): Revision Note

Antigens & Antibodies

• All cells have proteins and other substances projecting (sticking out) from their cell membrane

• These are known as antigens and are specific to that type of cell

• Lymphocytes have the ability to ‘read’ the antigens on the surfaces of cells and recognise any that are foreign

• They then make antibodies which are a complementary shape to the antigens on the surface of the pathogenic cell

• Antibodies can bind to antigens because of their complementary shapes

  • Like jigsaw puzzle pieces fitting together

Antigens and Antibodies Diagram

Antigens and antibodies

• The antibodies attach to the antigens and cause agglutination (clumping together)

• This means the pathogenic cells cannot move very easily

• At the same time, chemicals are released that signal to phagocytes that there are cells present that need to be destroyed

Agglutination of Pathogens Diagram

Agglutinated pathogens cannot move easily

• The initial response of a lymphocyte encountering a pathogen for the first time and making specific antibodies for its antigens can take a few days, during which time an individual may become ill

• Lymphocytes that have made antibodies for a specific pathogen for the first time will then make ‘memory cells’ that retain the instructions for making those specific antibodies for that type of pathogen

• This means that in the case of reinfection by the same type of pathogen, antibodies can be made very quickly, in greater quantities

  • The pathogens get destroyed before they are able to multiply and cause illness

• This is how people can become immune to certain diseases after only having them once

• It does not work with all disease-causing microorganisms as some of them mutate fairly quickly and change the antigens on their cell surfaces

• Therefore, if they invade the body for a second time, the memory cells made in the first infection will not recall them as they now have slightly different antigens on their surfaces (e.g. the cold virus)