Revision Notes

10.2 Immunity

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Antigens & Antibodies

  • All cells have proteins and other substances projecting 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


Antigens and antibodies, IGCSE & GCSE Biology revision notesAntigens 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


Agglutinated pathogens cannot move easily, IGCSE & GCSE Biology revision notesAgglutinated 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 get sick
  • 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 very quickly be made in greater quantities and the pathogens 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)
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Active & Passive Immunity

Active immunity:

  • Making antibodies and developing memory cells for future response to infection is known as active immunity
  • There are two ways in which this active immune response happens:
    • The body has become infected with a pathogen and so the lymphocytes go through the process of making antibodies specific to that pathogen
    • Vaccination
  • Active immunity is slow acting and provides long-lasting immunity


Passive immunity:

  • This is when ready-made antibodies, from another source, are introduced to the body
  • Passive immunity is a fast-acting, short-term defence against a pathogen by antibodies acquired from another individual, eg:
  • From mother to infant via breast milk – this is important as it helps the very young to fight off infections until they are older and stronger and their immune system is more responsive
  • Injected antibodies for certain diseases where the individual is already infected and a fast response is required, like rabies or tetanus
  • The body does not make its own antibodies or memory cells in passive immunity, hence the name
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Diseases Caused by the Immune System

  • Occasionally, the cells of the immune system start to attack the body’s own cells
  • This is rare as lymphocytes usually recognise their own body cells by the antigens on the cell surfaces and do not respond to them
  • In this situation, specific body cells are targeted by lymphocytes and antibodies are made against them, destroying them
  • One example of this type of disease is Type 1 diabetes
  • People who suffer from this disease no longer make their own insulin and so are unable to regulate their blood glucose levels
  • This is because their immune system is targeting and destroying the pancreatic cells which are responsible for making the insulin, eventually leading to dangerously high glucose levels in the blood

Exam Tip

There is often a lot of confusion amongst students regarding the terms antigen, antibody and antibiotic:

An antigen is a chemical found on the surface of a cell.
An antibody is a chemical made by lymphocytes that is complementary to an antigen and, when attached, clumps them together and signals the cells they are on for destruction.
An antibiotic is a drug that slows down or stops the growth of bacteria.

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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