Infection and response — AQA GCSE Biology Revision
This topic explores how pathogens, including viruses, bacteria, fungi, and protists, cause infectious diseases and how the human body defends itself agains
Topic Synopsis
This topic explores how pathogens, including viruses, bacteria, fungi, and protists, cause infectious diseases and how the human body defends itself against these threats. It covers the mechanisms of the immune system, the role of vaccinations, and the development and use of antibiotics and other medicines to treat disease.
Key Concepts & Core Principles
- Pathogens are microorganisms that cause infectious diseases; they can be bacteria, viruses, fungi, or protists. Each type has a different structure and mode of action, e.g., viruses reproduce inside host cells, while bacteria produce toxins.
- The immune system defends the body using white blood cells: phagocytes engulf pathogens, lymphocytes produce antibodies to neutralise specific antigens, and antitoxins counteract bacterial toxins. Memory cells remain after infection for faster response upon reinfection.
- Vaccination involves introducing a dead or weakened pathogen (or its antigens) to stimulate the immune system to produce memory cells without causing disease. This provides active immunity and helps prevent epidemics through herd immunity.
- Antibiotics, such as penicillin, kill or inhibit bacteria without harming human cells. They are ineffective against viruses. Overuse has led to antibiotic resistance, where bacteria evolve to survive treatment, e.g., MRSA.
- Drug development includes discovery (e.g., from plants or microorganisms), preclinical testing on cells and animals, and clinical trials on humans in phases to test safety, efficacy, and dosage. Placebos and double-blind trials reduce bias.
Exam Tips & Revision Strategies
- Ensure you can clearly distinguish between the different types of pathogens (viral, bacterial, fungal, protist).
- When describing the immune system, use the specific terms: phagocytosis, antibodies, and antitoxins.
- Be prepared to interpret data regarding the effectiveness of antibiotics or the spread of disease.
- Remember that viruses live inside cells, which makes them difficult to target with drugs without damaging the host's cells.
- Always link the use of vaccinations to the stimulation of white blood cells to produce antibodies.
Common Misconceptions & Mistakes to Avoid
- Confusing the action of antibiotics (which kill bacteria) with the action of painkillers (which only treat symptoms).
- Believing that antibiotics can kill viruses.
- Failing to distinguish between the roles of antibodies and antitoxins.
- Misunderstanding the role of vaccination in preventing rather than curing an active infection.
- Confusing the spread of pathogens (e.g., by air, water, or direct contact) with the pathogens themselves.
Examiner Marking Points
- Pathogens are microorganisms that cause infectious disease.
- Bacteria reproduce rapidly and may produce toxins that damage tissues.
- Viruses live and reproduce inside cells, causing cell damage.
- Non-specific human defence systems include skin, nose, trachea, bronchi, and stomach.
- White blood cells defend against pathogens via phagocytosis, antibody production, and antitoxin production.
- Vaccination involves introducing dead or inactive pathogens to stimulate antibody production.
- Antibiotics kill infective bacteria but do not kill viral pathogens.
- Painkillers treat symptoms but do not kill pathogens.