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Reflections and exam tips

Disease and Immunity

Disease

Is a state of unwellness, comes from 2 roots:

dis = not

ease = calmness

Diseases can be caused by pathogens (germs- don't use this term in exams), shortage of essential nutrients from a diet, physical damage to organs or even an imbalances of the body systems such as the nervous system.

Pathogens- a pathogen is a microorganism that causes a disease. Note that some microbes do not cause diseases. We eat some of them (mushrooms, bacteria in natural yoghurt and yeast used to make beer and bread).

Diseases caused by pathogens are usually infectious (spreadable).

Your need to know (name the parts) the structure of each of the 3 main pathogenic microbes; bacteria, viruses and fungi and explain how they cause disease.

The 3 main types of pathogens:

Bacteria

Structure

Bacteria are made up of cells, which can be a variety of shapes ranging from circular, rod-shaped to comma-shaped.

They have:

• a cell wall
• cell membrane
• cytoplasm
• small ribosomes
• DNA, lying naked in the cytoplasm

They belong to a group of organisms called Prokaryotes, organisms without a true nucleus. This means that they are very simple, with no membrane-bound organelles like mitochondria.

How they cause disease

• They can cause physical damage to the cell
• Produce toxins which damage cells and cause the signs of infection

Viruses

Structure

Viruses are not made up of cells. They are made of:

• a protein coat surrounding
• DNA at the centre
• tails to anchor onto their target

Viruses are:

• very tiny
• Smaller than bacteria
• Not made up of cells

How they cause disease:

Viruses cause disease by the way they reproduce:

• They inject their DNA into the cell
• Take over the nucleus
• Direct the cell to make more copies of themselves
• The cell bursts releasing more copies of themselves to infect more cells

The problem with visruses

• Viruses live inside cells, so they are difficult to treat/kill without damaging the cells.
• Most viruses mutate constantly- This means that they change their antigens (proteins) so medicines and antibodies can not easily recognise them.

Fungi

Structure

Fungi are made up of cells. Some are unicellular and some are multicellular. They can be spherical cells like yeast while some are fibrous hyphae, like bread mould.

They have:

• a cell wall
• cell membrane
• cytoplasm
• nucleus
• mitochrondria
• ribosomes

How they cause disease

• Fungi can be parasitic, growing into cells and extracting nutrients
• They can also produce toxins

Immunity

Means resistance to an infection.

External defences

The body has natural barriers which stop pathogens from going into cells. These are:

• The skin- a physical barrier around the body, acts like a fence
• Blood clotting- platelets in the blood help seal off a cut, preventing entry of pathogens
• Saliva- has lysozyme, an enzyme which can breakdown pathogens
• Tears - have lysozyme, an enzyme which can breakdown pathogens
• Mucus- sticky liquid in the airways, helps trap microbes
• Hairs and Cilia in airways- hairs trap dust and microbes while cilia sweeps away mucus
• Stomach acid- Hydrochloric acid destroys pathogens that have been taken in through the mouth.

Internal defences

These depend on the ability of the body to recognise foreign substances coming in. Each pathogen has protein markers called antigens on its surface. These help the body to detect if an invader has come in.

Internal defences work if microbes have managed to enter the body systems and cells:

These depend on the blood which is made up of:

• Red blood cells
• White blood cells
• Platelets
• Plasma

The white blood cells play a major role in immunity. There are 2 types of white blood cells:

• Lymphocytes
• Phagocytes

Lymphocytes produce ptoteins called antibodies, which stick to antigens on the pathogen's surface. This may either:

• kill the pathogens or
• make them clump together making them easy to destroy by phagocytes.
• Lymphocytes also produce antitoxins which neutralise poisons produced by pathogens.

Besides this, lymphocytes produce memory cells which are stored in the body. These already recognise the pathogen, if it comes back again, they produce antibodies faster and in large amounts before the pathogen causes infection.

Phagocytes detect the pathogen's antigen, engulf pathogens. This means that they surround, take in and digest them.

Types of immunity

1. Natural immunity:

When you catch an infection, your body produces antibodies and memory cells which protect you against future infections.

Babies also acquire antibodies from their mothers during breastfeeding.

2. Artificial immunity- Vaccination

This is sometimes called immunisation or vaccination.

During vaccination, weakened (attenuated) or dead pathogens are injected into the body. Sometimes only antigens from the pathogen are used. The body reacts by producing antibodies and memory cells which protect you from real infections later.

Semmelweis- 'The saviour of mothers'

Ignaz Semmelweis was a Hungarian doctor working at Vienna General Hospital. He showed how handwashing with disinfectant could reduce the transfer of pathogens from one patient to another. He hypothesized that doctors were routinely transmitting pathogens from dissecting corpses to new mothers as they did not wash their hands. As such mothers were dying of childbirth fever. This was in contrast to low death rates in wards which were worked by nurses only who did not handle dead bodies. Semmelweis then advised his trainees to wash their hands in disinfectant before helping mothers give birth. This was followed by a significant reduction in the death rates.

Why is Semmelweis's work still important?

• Semmelweis's work was the origin of the routine hand scrubbing which doctors practise.
• The technique of handwasing and the use of hand sanitizers has significantly reduced the spread of antibiotic resistant bacteria such as MRSA in hospitals.

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Challenge yourself:

1.Find out about MRSA. What is it? Why is it a problem.

2. What is antibiotic resistance? How does it arise. Use the concept of Natural selection to explain how antibiotic resistance arises.

3. Write a 6 mark essay explaining how white blood cells protect us from infection.