What you'll learn
- How plants defend themselves using physical barriers and chemical defences.
- How plant diseases can be detected and identified, especially in Separate Biology.
- How the human body stops pathogens entering and destroys them if they get inside.
- How immunisation and herd immunity reduce the spread of infectious disease.
The big idea: defence against pathogens
A pathogen is a microorganism that causes disease. Examples include bacteria, viruses, fungi and protists. A pest is an organism that damages another organism, for example an insect eating plant leaves.
Both plants and humans are constantly exposed to pathogens. To survive, they need:
- barriers to stop pathogens entering
- defences to kill or slow pathogens
- in humans, a specific immune response that targets particular pathogens
Pathogen
A pathogen is a microorganism that causes disease.
Plant defences
Plants cannot move away from pests or pathogens, so they rely on built-in defences. Some are always present, while others are produced when the plant is attacked.
If you are taking Separate Biology, you need to know the plant defence content in this section.
Physical barriers in plants
A physical barrier is a structure that blocks entry or makes it harder for pathogens or pests to get inside.
Plants use physical barriers such as:
- the leaf cuticle — a waxy waterproof layer on the surface of leaves
- the cell wall — a strong outer layer around plant cells, made mainly of cellulose
The leaf cuticle helps stop pathogens entering through the leaf surface. The cell wall makes it harder for pathogens to get into individual plant cells.
Plant physical defences
Plants defend themselves against pests and pathogens using barriers such as the waxy leaf cuticle and cellulose cell walls.
Chemical defences in plants
A chemical defence is a substance made by an organism that helps protect it.
Plants can produce chemicals that:
- kill pathogens
- slow pathogen growth
- taste unpleasant to pests
- poison or deter pests
Some plant chemicals are useful to humans because they can treat disease or relieve symptoms. For example, some medicines originally came from chemicals made by plants.
Plant chemicals are not made for humans
Plants produce defensive chemicals to protect themselves. Humans may discover and use some of these chemicals as medicines, but that is not why the plant makes them.
Detecting and identifying plant diseases
This part is Separate Biology, and the detailed disease-detection content is Higher Tier only.
A plant disease might be caused by a pathogen, but similar symptoms can also be caused by environmental problems such as mineral deficiency, drought, pollution or frost damage. So you should not jump straight to “pathogen” just because a plant looks unhealthy.
Visible symptoms
Symptoms are signs that show something may be wrong. In plants, visible symptoms can include:
- yellowing leaves
- brown spots or lesions
- wilting
- stunted growth
- rotting tissues
- abnormal growths
These symptoms help narrow down the possible causes, but they do not always prove the exact cause.
Eliminating environmental causes
Before identifying a pathogen, scientists may check for environmental causes, such as:
- shortage of mineral ions in the soil
- lack of water
- too much water
- exposure to chemicals
- temperature stress
For example, yellow leaves could be caused by a pathogen, but they could also be caused by a mineral deficiency.
Distribution analysis
Distribution analysis means looking at where affected plants are found.
If only plants in one dry area of a field are wilting, an environmental cause such as lack of water may be more likely. If diseased plants appear in spreading patches, a pathogen may be more likely.
Diagnostic testing
Diagnostic testing means using a test to identify the cause of disease. In the lab, scientists may test for a particular pathogen using samples from the affected plant.
Interpreting plant disease evidence
A gardener notices that tomato plants near a leaking pipe are yellow and stunted. Plants in other areas look healthy.
- The symptoms, yellowing and stunted growth, show the plants are unhealthy, but they do not prove a pathogen is present.
- The affected plants are clustered near the leaking pipe, so the distribution suggests an environmental cause linked to that location.
- Too much water around the roots could reduce oxygen availability for root cells, affecting mineral uptake and growth.
- A sensible conclusion is that waterlogged soil is more likely than a spreading pathogen, although further testing could be used if disease is still suspected.
Human body defences: stopping pathogens entering
The human body also has defences. The first line of defence is non-specific, meaning it acts against many pathogens in the same general way.
Skin
The skin is a physical barrier. It covers the body and stops many pathogens entering the blood and tissues. If skin is cut, pathogens can enter more easily.
Skin also produces antimicrobial substances, which help reduce pathogen growth on its surface.
Mucus and cilia
Mucus is a sticky substance that traps pathogens and dust in parts of the breathing system.
Cilia are tiny hair-like structures on some cells lining the airways. They waft mucus away from the lungs towards the throat, where it can be swallowed.
Cilia
Cilia are tiny hair-like structures on some cells that move substances, such as mucus, along surfaces.
Lysozymes and hydrochloric acid
Some human defences are chemical.
Lysozymes are enzymes found in body fluids such as tears and saliva. They destroy certain bacteria by breaking down their cell walls.
Hydrochloric acid is found in the stomach. It kills many pathogens that are swallowed in food, drink or mucus.
Human non-specific defences
Skin, mucus, cilia, lysozymes and stomach acid reduce the chance that pathogens enter the body or survive inside it.
Mucus does not kill most pathogens directly
Mucus mainly traps pathogens. Cilia then move the mucus away from the lungs. Chemical substances such as lysozymes and stomach acid are more directly involved in killing pathogens.
The specific immune system
If a pathogen gets past the first line of defence, the immune system responds.
The specific immune system targets particular pathogens. It recognises molecules on the surface of pathogens called antigens.
Antigen
An antigen is a molecule on the surface of a cell or pathogen that is recognised by the immune system.
Lymphocytes and antibodies
A lymphocyte is a type of white blood cell involved in the specific immune response.
When a lymphocyte recognises a pathogen’s antigen, it can produce antibodies. Antibodies are proteins that bind to specific antigens. This binding helps destroy the pathogen or mark it for destruction by other white blood cells.
Antibody
An antibody is a protein made by lymphocytes that binds specifically to a particular antigen.
The word specific is important. An antibody that fits one antigen usually will not fit a different antigen.
Antibody specificity
Antibodies are a bit like keys: each type has a shape that fits a particular antigen. A key that fits one lock will not necessarily fit another.
Memory lymphocytes and the secondary response
When your immune system first meets a pathogen, the response is called the primary response. It takes time because the correct lymphocytes must be activated and multiply.
Some activated lymphocytes become memory lymphocytes. These remain in the body after the infection has been cleared.
If the same antigen enters the body again, memory lymphocytes recognise it quickly. This causes a faster, stronger secondary response, producing antibodies much more rapidly.
Immune memory
Memory lymphocytes allow the body to respond faster if the same pathogen enters again.
Explaining a faster second response
A person catches a virus, recovers, and later meets the same virus again. They do not become very ill the second time.
- During the first infection, antigens on the virus triggered lymphocytes to produce antibodies.
- Some lymphocytes became memory lymphocytes specific to that virus antigen.
- On the second exposure, the memory lymphocytes recognised the antigen quickly.
- Antibodies were produced faster and in larger amounts, so the virus was destroyed before it caused serious symptoms.
Immunisation
Immunisation is the process of making someone immune to a disease, usually by vaccination.
A vaccine contains an inactive form of a pathogen, or parts of a pathogen, that cannot cause the full disease. However, it still has antigens that trigger an immune response.
The body responds by producing:
- antibodies
- memory lymphocytes
Later, if the real active pathogen enters the body, memory lymphocytes trigger a rapid secondary response.
Vaccine
A vaccine contains inactive or weakened pathogen material, or pathogen antigens, used to trigger immune memory without causing the full disease.
How to explain vaccination
Use this chain: inactive pathogen → antigens detected → antibodies made → memory lymphocytes made → faster response next time.
Advantages and disadvantages of immunisation
This discussion is Separate Biology content.
Advantages
Immunisation can:
- protect an individual from serious disease
- reduce the spread of pathogens through a population
- reduce the number of people needing hospital treatment
- help protect people who cannot be vaccinated
Herd immunity
Herd immunity happens when a high proportion of a population is immune to a disease. This makes it harder for the pathogen to spread because there are fewer susceptible people to infect.
Herd immunity helps protect people who cannot be immunised, such as some people with medical conditions or very young babies.
Herd immunity
Herd immunity is protection of a population when enough people are immune, reducing transmission of a pathogen.
Disadvantages
Immunisation also has possible disadvantages:
- vaccines may cause mild side effects, such as soreness or fever
- very rarely, more serious side effects can occur
- not every person develops full immunity
- some vaccines need booster doses
- high uptake is needed for herd immunity to work well
Comparing vaccination coverage
Two schools have the same number of students. In School A, 72 out of 100 students are vaccinated. In School B, 94 out of 100 students are vaccinated.
- Compare the proportion immune in each school: School A has 72% vaccinated, while School B has 94% vaccinated.
- A pathogen entering School B is more likely to meet immune people, so it has fewer chances to spread.
- School B is therefore more likely to have herd immunity than School A, assuming the vaccine is effective.
Herd immunity is not perfect protection
Herd immunity reduces spread, but it does not guarantee that no one will become infected. It depends on vaccine effectiveness, how easily the pathogen spreads, and how many people are immune.
In the exam
- For plant defences, separate physical barriers such as cuticle and cell wall from chemical defences such as antimicrobial chemicals or toxins.
- For immunity questions, always link antigen, lymphocyte, antibody and memory lymphocyte in the correct order.
- For vaccination, explain that the vaccine contains an inactive form of a pathogen that triggers memory lymphocytes, causing a faster secondary response later.
Check yourself
- How do the leaf cuticle and cell wall help protect plants from pathogens?
- Why is the secondary immune response faster than the primary response?
- How can immunisation of many people protect someone who has not been vaccinated?
