Health issues: Defects in the immune system mean that an individual is more likely to suffer from infectious diseases.

The idea

The immune system defends the body against pathogens. White blood cells, including lymphocytes, recognise antigens on pathogens and produce antibodies, while phagocytes engulf and digest microorganisms. If any part of this system is defective, the body cannot destroy pathogens effectively, so infections occur more often and may be more severe. Defects can be inherited, as in severe combined immunodeficiency, or acquired, as when HIV infects and destroys lymphocytes. Some medical treatments, such as chemotherapy or immunosuppressant drugs after an organ transplant, also reduce immune function. A person with a defective immune system may suffer repeated infections, take longer to recover, and be at greater risk from opportunistic pathogens that would normally be harmless.

What earns the marks

• The immune system involves white blood cells such as lymphocytes and phagocytes that recognise and destroy pathogens.
• A defect in the immune system reduces the body's ability to recognise or destroy pathogens, so infectious diseases are more likely.
• Named examples include HIV, which destroys lymphocytes, and inherited conditions such as severe combined immunodeficiency.
• Medical treatments such as chemotherapy and immunosuppressant drugs can weaken the immune system and increase the risk of infection.
• The consequence of a defective immune system is that infections may be more frequent, more severe, or caused by opportunistic pathogens.

Common mistakes and how to put them right

• Confusing a defect in the immune system with a defect in the nervous system.

  Put it right: The immune system involves white blood cells and defences against pathogens, not nerve impulses.

• Thinking that a weakened immune system directly causes infectious disease.

  Put it right: Infectious diseases are caused by pathogens; a weakened immune system increases the likelihood that pathogens will cause disease.

• Believing that all white blood cells produce antibodies.

  Put it right: Lymphocytes produce antibodies, whereas phagocytes engulf and digest pathogens.

Exam tips

• Name the relevant white blood cells and state their role when explaining why a defect increases infection risk.
• Use a specific example such as HIV destroying lymphocytes, and link this to a named infection such as tuberculosis.
• If the question asks about a treatment, explain how the treatment reduces immune function rather than simply naming the treatment.