Communicable Disease and Defence
This topic covers the distinction between communicable diseases, caused by pathogens and fought with treatments like vaccines, and non-communicable diseases, which arise from multiple factors and require long-term management.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Communicable diseases are caused by four types of pathogens: bacteria, viruses, fungi, and protists.
- Vaccines stimulate a primary immune response by introducing dead or inactive pathogens. This leads to the production of antibodies and memory cells, enabling a much faster and stronger secondary response upon future infection.
- HIV is a virus transmitted via bodily fluids that attacks the body's lymphocytes (a type of white blood cell). This severely weakens the immune system, leading to Acquired Immune Deficiency Syndrome (AIDS).
- Antibiotics are drugs used to treat bacterial infections only; they are ineffective against viruses, fungi, or protists.
- Non-communicable diseases, such as cardiovascular disease and type 2 diabetes, are not infectious and result from a combination of genetic, lifestyle, and environmental factors.
- Cardiovascular disease can be managed through medication (e.g., statins to lower cholesterol), surgical procedures (e.g., stents to open arteries), and lifestyle adjustments (e.g., diet and exercise).
Definitions
- Pathogen
- A microorganism, such as a bacterium or virus, that can cause disease.
- Antibody
- A protein produced by lymphocytes in response to a specific antigen (pathogen marker), which helps to neutralise the pathogen.
- Memory Cell
- A long-lived lymphocyte capable of responding very quickly to a second encounter with the same pathogen.
- Clinical Trial
- A multi-stage process of testing new medicines on human volunteers to assess their safety and effectiveness. It follows pre-clinical testing on cells and animals.
Worked example
The graph below shows the mean antibody concentration in the blood of two groups of people following an event at Week 4. Group X received a new vaccine at Week 0, while Group Y received a placebo. Both groups were exposed to a live pathogen at Week 4. Which statement is the most valid conclusion from this data? [Imagine a graph with Time (weeks) on the x-axis and Antibody Concentration (arbitrary units) on the y-axis. Group X's line shows a small peak after Week 0, then a very large, rapid peak after Week 4. Group Y's line is flat until Week 4, after which it shows a small, slow-rising peak that is much lower than Group X's second peak.] A) The vaccine is ineffective as Group X still produces antibodies after exposure at Week 4. B) The response in Group Y after Week 4 demonstrates the action of memory cells. C) The response in Group X after Week 4 is faster and larger than in Group Y due to a secondary immune response. D) Both groups show an identical primary immune response to the pathogen.
- 1
Analyse Group X's response:
A small antibody peak occurs after the vaccination at Week 0.
This is the primary immune response, creating memory cells.
After exposure to the live pathogen at Week 4, a much larger and faster antibody production occurs.
This is the secondary immune response.
- 2
Analyse Group Y's response:
This group received a placebo, so there is no antibody production before Week 4.
After exposure at Week 4, they mount a slow and relatively small antibody response.
This is their primary immune response to the pathogen.
- 3
Evaluate option A:
The large antibody response in Group X after exposure shows the vaccine is highly effective, not ineffective.
This is incorrect.
- 4
Evaluate option B:
Group Y had no prior exposure (placebo), so their response after Week 4 is a primary response, not one involving memory cells.
This is incorrect.
- 5
Evaluate option C:
Comparing the peaks after Week 4, Group X's response is clearly faster (steeper slope) and larger (higher peak) than Group Y's.
This is characteristic of a secondary immune response, which is the purpose of vaccination.
This statement is correct.
- 6
Evaluate option D:
The primary response of Group Y (after Week 4) is much smaller than the secondary response of Group X (after Week 4).
They are not identical.
This is incorrect.
Answer: C) The response in Group X after Week 4 is faster and larger than in Group Y due to a secondary immune response.
Common mistakes
- ×Misinterpreting data from graphs, especially confusing the primary immune response (slow, small) with the secondary response (fast, large) or misreading axes.
- ×Confusing the function of antibiotics and vaccines. Remember: Antibiotics TREAT existing bacterial infections. Vaccines PREVENT future infections.
- ×Mistaking HIV for AIDS. HIV is the virus that causes the infection. AIDS is the late-stage condition where the immune system is severely compromised.
No-calculator tips
- ✓In drug trial questions, focus on relative changes and comparisons. Is one group's success rate double the other? Is the effect immediate or delayed? You don't need to calculate precise percentages.
- ✓Remember the sequence of drug development: Pre-clinical (cells, animals) → Clinical Phase 1 (small group, safety) → Phase 2 (medium group, effectiveness) → Phase 3 (large group, confirm results). No calculation is needed, just logical ordering.