How pathogens cause disease and how the immune system fights back
Health is a state of complete physical, mental and social well-being. Infectious diseases are caused by pathogens. Bacteria cause typhoid and pneumonia, viruses cause the common cold and AIDS, protozoans such as Plasmodium cause malaria, and helminths such as Ascaris cause ascariasis. Malaria’s life cycle passes between humans and female Anopheles mosquitoes, and fever recurs when infected red blood cells burst and release haemozoin.
Immunity defends the body. Innate immunity includes barriers like skin, mucus, phagocytes and fever. Acquired immunity is specific and has memory. B lymphocytes produce antibodies in humoral immunity, and T lymphocytes drive cell-mediated immunity. Antibodies are Y-shaped proteins with two heavy and two light chains. Vaccination creates memory cells so later infections are fought quickly. Allergies are overreactions to harmless substances, and autoimmunity is an attack on the body’s own cells. HIV attacks helper T cells. Cancer arises when cells divide uncontrollably. Drugs, tobacco and alcohol damage health and can cause addiction.
Why it matters
Understanding pathogens and immunity explains why vaccines have greatly reduced diseases such as polio and measles, and why antibiotics do not work against viruses. Public health measures, including clean water, sanitation and mosquito control, prevent epidemics. Knowledge of drug and alcohol abuse helps young people make safer choices.
Key terms
- Pathogen
- An organism such as a bacterium, virus or protozoan that causes disease.
- Antibody
- A protein made by B cells that binds to a specific antigen to neutralise it.
- Active immunity
- Immunity developed when the body produces its own antibodies after infection or vaccination.
- Vaccination
- Introducing a weakened or inactivated pathogen or its parts to build immune memory.