Defects in the immune system mean that an individual is more likely to…

The idea

The immune system defends the body against pathogens such as bacteria, viruses, fungi and protists. White blood cells engulf pathogens and produce antibodies or antitoxins. If any part of this defence is defective, pathogens are not destroyed efficiently, so they multiply and cause infectious disease more often and often more severely. Defects may be inherited, such as severe combined immunodeficiency, or acquired, for example through infection with HIV, which destroys helper T cells, or through drugs that suppress immunity after a transplant. A student can reason from cause to effect: fewer functional white blood cells means weaker phagocytosis and antibody production, so a common pathogen such as Mycobacterium tuberculosis is more likely to establish infection. This links immune function to health, disease transmission and vaccination.

What earns the marks

• The immune system uses white blood cells to engulf pathogens and to produce antibodies and antitoxins that destroy pathogens or neutralise their toxins.
• A defect means one or more of these defences work less effectively, for example too few functional white blood cells or reduced antibody production.
• Because pathogens are not destroyed efficiently, they reproduce inside the body and the individual develops an infectious disease more readily.
• Defects can be inherited, caused by a pathogen such as HIV, or caused by medical treatment that suppresses the immune system.
• The consequence is increased frequency and often increased severity of infections, not a direct cause of non-infectious diseases such as scurvy.
• A valid explanation links the defective defence mechanism to pathogen survival and multiplication, then to the symptoms of infectious disease.

Common mistakes and how to put them right

• Saying a defective immune system directly causes infectious diseases.

  Put it right by stating that pathogens cause the disease and the defect only increases susceptibility.

• Confusing a weak immune system with an autoimmune reaction.

  Put it right by distinguishing reduced defence against pathogens from the immune system attacking the body's own cells.

• Claiming that all infections in an immunocompromised person are caused by a new mutation.

  Put it right by explaining that ordinary environmental pathogens are simply not controlled.

• Describing antibodies as cells.

  Put it right by stating that antibodies are proteins produced by white blood cells.

Exam tips

• Use the chain 'defect → reduced phagocytosis or antibody production → pathogens survive and multiply → infectious disease' to structure an explanation.
• Name a specific defect or cause, such as HIV destroying helper T cells, to show understanding beyond a general statement.
• When asked to explain rather than state, include the mechanism of white blood cell action before linking it to increased infection risk.
• Check that every clause of the question is addressed, including both the defect and the increased likelihood of infectious disease.