4.3 Infection and Response

Health

Health is the state of physical and mental well-being. Diseases, both communicable and non-communicable, are major causes of ill health. Other factors including diet, stress and life situations such as working with chemicals may have a profound effect on both physical and mental health.

Communicable diseases

Communicable diseases are those that can spread from person to person or between animals and people. They can be caused by things like bacteria, viruses, parasites and fungi. Measles and malaria are examples of communicable diseases.

Non-communicable diseases

Non-communicable diseases are those that cannot spread from person to person or between animals and people. They generally last for a long time and slowly get worse overtime. Asthma, cancer and coronary heart disease are examples of non-communicable diseases.

Pathogens

Communicable diseases are spread by pathogens. Pathogens are microorganisms that cause infectious disease. Pathogens include bacteria, viruses, protists and fungi.

Bacteria

Bacteria can be fatal, once inside the human body they rapidly reproduce and under ideal conditions bacteria can divide every 20 minutes. They can release harmful chemicals called toxins which damage tissues and can make us ill.

Viruses

Viruses cannot reproduce by themselves, they can only reproduce in a host cell. The virus invades the host cell and when it reproduces it is very damaging to the cell as it can cause the cell to burst open and die.

Spreading of pathogens

Pathogens are usually spread in three ways, some pathogens are spread in the air (in water droplets) for example influenza, directly through the water for example cholera by drinking water polluted by sewage, and direct contact between individuals for example HIV from needles or sexual contact. The spread of pathogens can be prevented with basic hygiene (washing hands before eating) and providing people with clean drinking water so in the UK drinking water contains chlorine which kills microbes, and reducing direct contact between individuals for example using a condom during sexual intercourse. In the case of highly infectious diseases such as Ebola, patients may be isolated preventing the pathogen from spreading to other people. Vaccination also reduces the spread of pathogens as vaccines help build a resistance against pathogens.

Measles - A viral disease

Measles is a highly infectious viral disease showing symptoms of fever and a red skin rash. Measles is a serious illness that can be fatal if complications arise such as damage to the breathing system and the brain. For this reason most young children are vaccinated against measles. The measles virus is spread by inhalation of droplets from sneezes and coughs.

HIV - A viral disease

HIV initially causes a flu-like illness. Unless successfully controlled with antiretroviral drugs the virus attacks the body’s immune cells damaging the immune system. Late stage HIV infection, or AIDS, occurs when the body’s immune system becomes so badly damaged it can no longer fight off with other infections that most people can and is unable fight off cancers cells. At this stage the disease can be fatal. HIV is spread by sexual contact or exchange of body fluids such as blood which occurs when drug users share needles. Antiretroviral drugs stop the virus from multiplying inside the patient so the virus doesn't damage the patient's immune system. Patients who take antiretroviral drugs do not go on to develop AIDS and they can lead a normal life and have a normal life expectancy. Antiretroviral drugs aren't a cure for HIV or AIDS, the patient must take these drugs for the rest of their life.

Tobacco mosaic virus - A viral disease

Tobacco mosaic virus (TMV) is a widespread plant pathogen affecting many species of plants including tomatoes. It gives a distinctive ‘mosaic’ pattern of discolouration on the leaves which affects the growth of the plant due to lack of photosynthesis.

Salmonella - A bacterial disease

Salmonella food poisoning is spread by bacteria ingested in food, or on food prepared in unhygienic conditions. In the UK, poultry are vaccinated against Salmonella to control the spread. Fever, abdominal cramps, vomiting and diarrhoea are caused by the salmonella bacteria secreting harmful bacteria and toxins. Salmonella bacteria are sometimes found in poultry such as chicken however in the UK all chicken are vaccinated against salmonella controlling the spread of the disease.

Gonorrhoea - A bacterial disease

Gonorrhoea is a sexually transmitted disease (STD) with symptoms of a thick yellow or green discharge from the vagina or penis and pain when urinating. It is caused by a bacterium and was easily treated with the antibiotic penicillin until many resistant strains appeared. Gonorrhoea is spread by sexual contact. The spread can be controlled by treatment with antibiotics or the use of a barrier method of contraception such as a condom. People should also test for STDs regularly and if they have gonorrhoea they should take a full course of antibiotics to kill the bacteria before they pass it onto another person.

Rose balck spot - A fungal disease

Rose black spot is a fungal disease where purple or black spots develop on leaves, which often turn yellow and drop early. It affects the growth of the plant as photosynthesis is reduced. It is spread in the environment by water or wind. Rose black spot can be treated by using fungicides and/or removing and destroying the affected leaves.

Protist diseases

The pathogens that cause malaria are protists. The malarial protist has a life cycle that includes the mosquito. The mosquito bites a person with malaria and the malaria pathogen passes into the mosquito. The mosquito then bites a different person passing the malaria pathogen to them. Here the mosquito is a vector as it has spread the pathogen from one person to another. Malaria causes recurrent episodes of fever and can be fatal. The spread of malaria is controlled by preventing the vectors, mosquitos, from breeding by draining areas of still water and spray these areas with insecticide as this is their breeding ground, and by using mosquito nets to avoid being bitten.

Non-specific defense systems of the human body against pathogens

Skin which provides a protective layer covering the body, the outer layer of the skin consists of dead skin cells and is difficult for pathogens to penetrate. The skin also produces an oily substance called sebum which can kill bacteria. Sometimes the skin is damaged which could allow pathogens to enter the body so to stop this the skin scabs over.

Penis and anus tubes are openings in the skin so pathogens can enter so they are slightly acidic killing any pathogens.

Nose, as this is an opening in the skin where pathogens can enter the body so the nose contains hairs and mucus which can trap pathogens before they enter the breathing system.

Trachea and bronchi, as some pathogens can make it in through the nose and mouth so the trachea and bronchi are also covered with tiny hairs called cilia. Cillia are trapped in mucus which can help trap pathogens. The cilia waft pathogens towards the throat where it is swallowed into the stomach.

HCl in stomach kills pathogens swallowed before they make their way further down into the digestive system.

The role of the immune system in defense against disease

Phagocytes are a type of white blood cell and these detect chemicals released from a pathogen, then chase, engulf and ingests pathogens before using enzymes to kill them. The entire process is called phagocytosis. Lymphocytes are another type of white blood cell and they make proteins called antibodies against the antigens on the pathogen neutralising it and triggering it to be destroyed. Antibodies are always complimentary to the antigen so one type of antibody cannot be used against a different type of pathogen. Antibodies stay in the bloodstream for a long time so they can protect humans against the same pathogen if it does infect the human again. Some pathogens such as the pathogen for salmonella bacteria release toxins that can harm the human body and cause humans to become ill so the lymphocytes produce antitoxins which bind to the toxins and neutralise them preventing them from damaging cells.

Vaccination

Vaccination involves introducing small quantities of dead or inactive forms of a pathogen into the body to stimulate the white blood cells to produce antibodies. Because the pathogen is dead or inactive it cannot lead to disease in the patient. As the lymphocytes are stimulated to make antibodies the white blood cell divides by mitosis to produce lots of copies of itself which stay in the blood for decades so if the same pathogen re-enters the body even years later, the white blood cells respond quickly to produce the correct antibodies, preventing infection.

Some people will always not be vaccinated and this can be because they are new to the country or they missed a vaccination appointment. As long as the majority of people are vaccinated, these unvaccinated people are protected as the unvaccinated person cannot catch the disease from a vaccinated person. This is called herd immunity and it requires 95% of the population to be vaccinated so it is difficult to achieve.

Antibiotics and painkillers

Antibiotics, such as penicillin, are medicines that help to cure bacterial disease by killing infective bacteria inside the body without harming the body cells. It is important that specific bacteria should be treated by specific antibiotics.

The use of antibiotics has greatly reduced deaths from infectious bacterial diseases as many people died from bacterial diseases but around the 1940s scientists discovered the first antibiotic, penicillin. However, the emergence of strains resistant to antibiotics is of great concern. Certain antibiotics are no longer effective against certain bacteria as antibiotics had been overused so the bacteria evolved to no longer be killed by the antibiotic, this is called antibiotic resistance. In the future it is possible that bacteria become very difficult to treat.

Antibiotics are specific to the bacteria they treat and antibiotics cannot kill viral pathogens. Painkillers and other medicines are used to treat the symptoms of disease but do not kill pathogens. It is difficult to develop drugs that kill viruses without also damaging the body’s tissues because viruses live and reproduce inside human body cells. This leads doctors to not overprescribe and be hesitant to prescribe antibiotics as some conditions may be caused by a virus instead.

What drugs originate from plants and microorganisms.

Traditionally drugs were extracted from plants and microorganisms.

The heart drug digitalis originates from foxgloves.

The painkiller aspirin originates from willow.

Penicillin was discovered by Alexander Fleming from the Penicillium mould.

Most new drugs are synthesised by chemists in the pharmaceutical industry. However, the starting point may still be a chemical extracted from a plant.

Development and testing of new drugs

New medical drugs have to be tested and trialled before being used to check that they are safe and effective. New drugs are extensively tested for toxicity (whether it is safe for humans), efficacy (how effective of a drug it is), and dose (the optimal amount of the drug to be given).

Preclinical testing is done in a laboratory using cells, tissues and live animals.

Clinical trials use healthy volunteers and patients.

1. Very low doses of the drug are given at the start of the clinical trial.

2. If the drug is found to be safe, further clinical trials are carried out to find the optimum dose for the drug.

3. In double blind trials, some patients are given a placebo (tablet or injection with no active drug in it, people believe they are being treated so they feel better even though the drug didn't do anything) so the patient do not know whether they have the drug or not, and the scientists also do not know which patients have the drug (preventing unconscious biases such as paying closer attention to the patients with the drug) making it a double blind trial rather than a blind trial.

How monoclonal antibodies are produced

Scientists can trigger lymphocytes to produce antibodies and this is useful, for example in pregnancy testing kits and in certain cancer treatments.

To produce antibodies, an antigen is injected into the mouse and the mouse's lymphocytes that make the specific antibody against the specific antigen are collected. The lymphocytes won't divide by mitosis so they are combined or fused with a tumour cell (as they are very good at dividing by mitosis) forming a hybridoma. Hybridoma cells can produce antibodies that divide by mitosis. A hybridoma that is producing the wanted antibody is selected, and is allowed to divide by mitosis to form clones of identical hybridoma cells. These identical hybridoma cells are called monoclonal antibodies, as they all come from a single hybridoma clone. A large amount of monoclonal antibodies can be collected and purified.

As monoclonal antibodies are produced from a single clone of hybridoma cells they are specific to one binding site on one protein antigen so they can target a specific chemical or specific cells in the body so they have a large number of uses.

Uses of monoclonal antibodies

Monoclonal antibodies can be made against any antigen.

 

In pregnancy testing monoclonal antibodies are used to detect a specific hormone which is produced by the placenta of the developing foetus. Pregnancy test kits based on monoclonal antibodies are cheap and easy to use, the woman simply urinates on the test strip and observes for the reaction. If used correctly the test is highly accurate.

 

Monoclonal antibodies are also used in labs, they can be used to measure the levels of hormones in blood and to detect pathogens in the blood such as viruses.

Monoclonal antibodies can be used to locate or identify specific molecules in a cell or tissue by being attached to fluorescent dyes and then stick to specific molecules within the cell allowing us to see their locations.

Cancer cells can undergo uncontrolled mitosis and they spread around the body. Antibodies specific to cancer cells can be made and a radioactive substance or toxic drug is attached to the antibody. The antibody attaches itself to the cancer cells and the radioactive substance or toxic drug stops the cancer cells from growing and dividing. The advantage here is that the antibody delivers the substance specifically to the cancer cells without harming other cells in the body.

In certain drug trials, monoclonal antibodies create more side effects than expected and these are very harmful. They are not yet as widely used as everyone hoped when they were first developed.

Plant diseases can be detected by

Stunted growth

Spots on leaves

Areas of decay (rot)

Growths

Malformed stems or leaves

Discolouration

The presence of pests.

Plant diseases can be identified by

reference to a gardening manual or website

taking infected plants to a laboratory to identify the pathogen

using testing kits that contain monoclonal antibodies.

What can plants be infected by?

Plants can be infected by a range of viral, bacterial and fungal pathogens as well as by insects such as aphids. Aphids extract nutrients such as sugars from the plant stunting its growth. Insects are not pathogens as they do not cause infectious disease although many insects carry pathogens such as viruses.

Ion deficiencies in plants

Plants can be damaged by a range of ion deficiency conditions including:

Stunted growth caused by nitrate deficiency - Nitrate is needed for protein synthesis and therefore growth

Chlorosis caused by magnesium deficiency - Magnesium is required to make chlorophyll

Infectious diseases for plants

Plants can contract infectious diseases.

TMV is a widespread plant infection that infects a number of plant species including tomatoes. TMV causes the leaves of a plant to discolour in a mosaic pattern and because of this discolouration the rate of photosynthesis is reduced so the growth of the plant is also reduced.

Rose black spot is caused by a fungus. RBS causes the leaves to develop purple or black spots before the leaf turns yellow and falls off. Just like TMV, RBS causes the rate of photosynthesis of the infected plant to drop reducing the rate of growth. The fungus RBS can be spread by water or wind. To treat RBS the infected and not infected plants can be sprayed with fungicides which are chemicals that kill fungi and prevent them from spreading. The other way is to trim off the infected leaves and destroy them.

What are the three plant defence systems?

Physical
Mechanical
Chemical

Plant Physical defence responses to resist invasion of microorganisms.

Cellulose cell walls - difficult for microorganisms such as bacteria to penetrate

Tough thin oily layer waxy cuticle on leaves - difficult for microorganisms to penetrate protecting the plant from attack

Layers of dead cells around stems (bark on trees) which fall off - bark is a barrier to entry for microorganisms, as bark ages it eventually falls off and is replaced with fresh bark underneath

Plant Chemical defence responses to resist invasion of microorganisms

Antibacterial chemicals - kill bacteria and prevent them attacking the plant

Poisons - deter herbivores grazing

Plant Mechanical adaptations

Thorns and hairs - deter animals and prevent being eaten and irritate the mouth of herbivores making it difficult to eat

Leaves which droop or curl when touched - scare herbivores as they aren't used to it

Mimicry to trick animals - the white dead nettle has no sting but looks very similar to a stinging nettle so herbivores mistake it as a plant that can sting