Antigens & Antibodies (Cambridge (CIE) IGCSE Biology): Revision Note

Antigens & antibodies: Extended

Extended Tier Only

• All cells have molecules, such as proteins, projecting from their cell membranes

• These are known as antigens

• Different individuals have different antigens on their cell surface membranes

• Lymphocytes can recognise foreign antigens, e.g. the antigens of a pathogen inside the body

• In response to foreign antigens lymphocytes make antibodies which are complementary in shape to the antigens on the surface of the pathogenic cell

The role of antibodies

• Antibodies can attach to antigens and cause agglutination of pathogens

  • This means the pathogenic cells cannot move around very easily

• At the same time chemicals are released that signal the presence of pathogens to phagocytes

• Phagocytes move towards the site of an infection where they engulf and destroy pathogens

  • Many pathogens can be engulfed together due to agglutination

Immunity

• The initial response of a lymphocyte encountering a pathogen for the first time can take a few days, during which time an individual may get sick

• After an initial encounter with a pathogen, lymphocytes can give rise to memory cells that retain the instructions for making specific antibodies

• This means that in the case of reinfection by the same type of pathogen, antibodies can be made very quickly and in greater quantities; the pathogens are destroyed before they multiply and cause illness

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

• Note that this does not work with all disease-causing microorganisms as some of them mutate quickly and change the antigens on their cell surface

  • Therefore, if they invade the body for a second time the memory cells made in the first infection will not produce antibodies that match the new antigens