Students should be able to explain the role of the immune system in the…

The idea

The immune system is the body's specific defence against pathogens that have breached the non-specific barriers. White blood cells patrol the blood and lymph. Phagocytes engulf and digest pathogens by phagocytosis, while lymphocytes produce antibodies that bind to antigens on a pathogen's surface, clumping pathogens so phagocytes destroy them. Lymphocytes also produce antitoxins that neutralise toxins released by bacteria. Memory cells remain after infection, giving long-lasting immunity and a faster, larger response on re-exposure.

For example: After measles infection, memory lymphocytes recognise measles antigens and destroy the virus before symptoms develop.

What earns the marks

• Phagocytes carry out phagocytosis: they engulf a pathogen and digest it using enzymes.
• Lymphocytes recognise specific antigens on a pathogen's surface and produce complementary antibodies.
• Antibodies bind to antigens, causing pathogens to clump so phagocytes can destroy them more easily.
• Lymphocytes produce antitoxins that neutralise toxins released by pathogens such as bacteria.
• Memory cells remain after an infection, so a later infection by the same pathogen triggers a faster, larger antibody response.
• The immune system is specific: antibodies and memory cells act against one particular antigen, not all pathogens.

Common mistakes and how to put them right

• Saying white blood cells 'eat' all pathogens indiscriminately.

  Put it right: Phagocytosis is non-specific, but antibody production by lymphocytes is specific to antigens.

• Confusing antibodies with antitoxins.

  Put it right: Antibodies bind antigens and clump pathogens, whereas antitoxins neutralise toxins.

• Stating that antibiotics are produced by white blood cells.

  Put it right: Antibodies and antitoxins are produced by lymphocytes, while antibiotics are medicines that kill bacteria.

Exam tips

• Name the white blood cell type and its job: phagocyte for engulfing, lymphocyte for antibodies and antitoxins.
• Use the words antigen and antibody correctly: antigens are on the pathogen surface, antibodies are produced by lymphocytes and bind to antigens.
• Link memory cells to immunity by explaining the faster, larger second response rather than just saying 'you are immune'.