Module 4: 3 - Communicable Diseases

Type 1 diabetes

Body Part Affected The insulin-secreting cells of the pancreas Treatment Insulin injections Pancreas transplants Immunosuppressant drugs

Rheumatoid arthritis

Body part affected: Joints - especially in the hands, wrists, ankles and feet Treatment: No cure Anti-inflammatory drugs Steroids Immunosuppressants Pain relief

Lupus

Body part affected: Often affects skin and joints and causes fatigue Can attack any organ in the body including kidneys, liver, lungs or brain Treatment: No cure Anti-inflammatory drugs Steroids Immunosuppressants Various others

Antigen presenting cells (APCs)

It usually takes a human neutrophil under 10 minutes to engulf and destroy a bacterium. Macrophages take longer but they undergo a more complex process. When a macrophage has digested a pathogen, it combines antigens from the pathogen surface membrane with special glycoproteins in the cytoplasm called the major histocompatibility complex (MHC). The MHC complex moves these pathogen antigens to the macrophage's own surface membrane, becoming an antigen-presenting cell (APC). These antigens now stimulate other cells involved in the specific immune system response.

Preventing the spread of communicable diseases in plants

Key factors in reducing the spread of communicable diseases in plants: - Leave plenty of room between plants to minimise spread of pathogens - Clear fields as thoroughly as possible - remove all traces of plants at harvesting - Rotate crops - the spores or bacteria will eventually die if they do not have access to the plant - Follow strict hygiene practices - measures such as washing hands, machinery, etc. - Control insect vectors

Fevers

Normal body temperature of around 37ºC is maintained by the hypothalamus in your brain. When a pathogen invades your body, cytokines stimulate your hypothalamus to reset the thermostat and your temperature goes up. This is a useful adaptation because: - Most pathogens reproduce best at or below 37ºC. Higher temperatures inhibit pathogen reproduction. - The specific immune system works faster at higher temperatures

Recognising an attack

Plants are not passive - they respond rapidly to pathogen attacks. Receptors in the cells respond to molecules from the pathogens, or to chemicals produced when the plant cell wall is attacked. This stimulates the release of signalling molecules that appear to switch on genes in the nucleus. This in turn triggers cellular responses, which include producing defensive chemicals, sending alarm signals to unaffected cells to trigger their defences, and physically strengthening the cell walls.

Plant Defences Against Pathogens

Plants have evolved a number of ways to defend themselves against the pathogens that cause communicable diseases. The waxy cuticle of plant leaves, the bark on trees and the cellulose cell walls of individual plant cells act as barriers, which prevent pathogens getting in. Unlike animals, plants do not heal diseased tissue - they seal it off and sacrifice it. Because they are continually growing at the meristems, they can then replace the damaged parts.

Solving the problem

The development of antibiotic-resistant bacteria is one of the biggest health problems of our time. Scientists are working on developing new antibiotics using computer modelling and looking are possible sources in a wide variety of places including soil microorganisms and crocodile blood. But at the moment, bacterial resistance is building faster than new antibiotics can be found.

Stages of phagocytosis

1) Pathogens produce chemicals that attract pathogens 2) Phagocytes recognises non-human proteins on the pathogen. This is a response not to a specific type of pathogen, but simply a cell or organisms that is non-self. 3) The phagocyte engulfs the pathogen and encloses it in a vacuole called a phagosome 4) The phagosome combines with a lysosome to form a phagolysosome. 5) Enzymes from the lysosome digest and destroy the pathogen.

What are the main steps of vaccination?

1) The pathogen is made safe in one of a number of ways so that the antigens are intact but there is no risk of infection. 2) Small amounts of the vaccine are injected into the blood 3) The primary immune response is triggered by the foreign antigens and your body produces antibodies and memory cells as if you were infected with a live pathogen. 4) If you come into contact with a live pathogen, the secondary immune response is triggered and you destroy the pathogen rapidly before you suffer symptoms of the disease The artificial active immunity provided by vaccines may last a year, a few years or a lifetime. Sometimes boosters (repeat vaccinations) are needed to increase the time you are immune to a disease.

Tobacco mosaic virus (TMV)

A virus that infects tobacco plants and around 150 other species including tomatoes and peppers. It damages leaves, flowers and fruit, stunting growth and reducing yields, and can lead to an almost total crop loss. Resistant crop strains are available but there is no cure.

Tuberculosis

A bacterial disease of humans, cows, pigs, badgers and deer commonly caused by Mycobacterium tuberculosis and M. bovis. TB damages and destroys lung tissues and suppresses the immune system so the body is less able to fight off other diseases. Worldwide in 2012 around 8.6 million people had TB of which 1.3 million people died. The global rise of HIV/AIDS has had a big impact on the numbers of people also suffering from diseases such as TB, because people affected by HIV/AIDS are much more likely to develop TB infections. In people TB is both curable (by antibiotics) and preventable (by improving living standards and vaccination).

Ring rot

A bacterial disease of potatoes, tomatoes and aubergines caused by Gram positive bacterium Clavibacter michiganensis. It damages leaves, tubers and fruit. It can destroy up to 80% of crop and there is no cure. Once the bacterial ring rot infects a field it cannot be used to grow potatoes again for at least 2 years.

Bacterial Meningitis

A bacterial infection of the meninges of the brain which can spread into the rest of the body causing septicaemia (blood poisoning) and rapid death. It mainly affects very young children and teenagers aged 15-19. They have different symptoms but in both, a blotchy red/purple rash that does not disappear when a glass is pressed against it is a symptom of septicaemia and immediate medical treatment is needed. About 10% of people infected will die. Up to 25% of those who recover will have some permanent damage. Antibiotics will cure the disease if delivered early. Vaccines can protect against some forms of bacterial meningitis.

Black sigatoka

A banana disease caused by the fungus Mycosphaerella fijiensis, which attacks and destroys the leaves. The hyphae penetrate and digest the cells, turning the leaves black. If plants are infected it can cause a 50% reduction in yield. Resistant strains are being developed - good husbandry and fungicide (a chemical that kills fungi) treatment can control the spread of the disease but there is no cure.

Zoonotic influenza

A disease that people can catch from animals is known as a zoonosis. Influenza for example attacks a range of animals including birds and pigs. Sometimes the virus which causes bird flu or swine (pig) flu mutates and becomes capable of infecting people. These new strains can be particularly serious, because few people have any natural immunity to them. In March 2009, 60% of the population of a small town in Mexico became infected with a new disease and two babies died. Some of those infected tested positive for H1N1, a form of flu usually found in pigs, rather than the usual human flu strains. The outbreak spread to the US, where using DNA analysis techniques, the virus was identified as a new mutant strain of the H1N1 swine flu virus, which had not been seen before in either pigs or people. Three months after it first appeared people were infected with H1N1 flu in 62 countries, and some of them were dying. None of the available flu vaccines were any use against this zoonotic virus. Within five months almost 3000 people around the world had died. Fortunately, only six months after swine flu H1N1 was first identified, scientists produced an effect vaccine. In spite of this, recent analyses of the data suggest between 200,000-300,000 people died as a result of H1N1 infection in the 2009 outbreak - and up to 80% of those deaths were in people aged 65 and younger. In a normal seasonal flu outbreak, only around 10% of deaths occur in people who are under 65. H1N1 is now part of the normal seasonal flu vaccine and scientists remain on the lookout for the next mutation which may enable the flu virus to pass from pigs or birds to people, known as a species jump.

Ring worm

A fungal disease affecting mammals including cattle, dogs, cats and humans. Different fungi infect different species - in cattle, ring worm is usually caused by Trichophyton verrucosum. It causes grey-white, crusty, infectious, circular areas of skin. It is not damaging but looks unsightly and may be itchy. Antifungal creams are an effective cure.

Athlete's Foot

A human fungal disease caused by Tinia pedia, a form of human ring worm that grows on and digests the warm moist skin between the toes. It causes cracking and scaling, which is itchy and may become sore. Antifungal creams are an effective cure.

Factors affecting the transmission of communicable diseases in plants

A number of factors are responsible: - Planting varieties of crops that are susceptible to disease - Over-crowding increases the likelihood of contact - Poor mineral nutrition reduces resistance of plants - Damp, warm conditions increase the survival and spread of pathogens and spores - Climate change - increased rainfall and wind promote the spread of diseases

Bacteria

A small proportion of these bacteria are pathogens, causing communicable diseases. Bacteria are prokaryotes, so they have a cell structure that is very different from the eukaryotic organisms they infect. They do not have a membrane-bound nucleus or organelles. Bacteria can be classified in two main ways: - By their basic shapes - they may be rod shaped (bacilli), spherical (cocci), comma shaped (vibrios), spiralled (spirilla) and corkscrew (spirochaetes). - By their cell walls - the two main types of bacterial cell walls have different structures and react differently with a process called Gram staining. Following staining Gram positive bacteria look purple-blue under the light microscope, for example MRSA. Gram negative bacteria appear red, for example E.coli. This is useful because the type of cell wall affects how bacteria react to different antibiotics (a compound that kills or inhibits the growth of bacteria.

Influenza (flu)

A viral infection (Orthomyxoviridae spp.) of the ciliated epithelial cells in the gas exchange system. It kills the, leaving the airways open to secondary infection. Flu can be fatal, especially to young children, old people and people with chronic illnesses. Many of these deaths are from severe secondary bacterial infections such as pneumonia on top of the original viral infection. Flu affects mammals, including humans and pigs, and birds, including chickens. There are three main strains - A, B and C. Strain A viruses are the most virulent and they are classified further by the proteins on their surfaces, for example A(H1N1) and A(H3N3). The viruses mutate regularly. The change is usually quite small, so having flu one year leaves you with some immunity for the next. Every so often, however, there is a major change in the surface antigens and this heralds a flu epidemic or pandemic as there are no antibodies available. Vulnerable groups are given a flu vaccine annually to protect against ever changing strains. There is no cure.

Antigens

All cells have molecules called antigens on their surfaces. The body recognises the difference between self antigens on your own cells and non-self antigens on the cells of pathogens. Some toxins also act as antigens. Antigens trigger an immune response, which involves the production of polypeptides called antibodies.

Synthetic biology

Another major step forward in drug development is synthetic biology. Using the techniques of genetic engineering, we can develop populations of bacteria to produce much needed drugs that would otherwise be too rare, too expensive or just not available. Synthetic biology enables the use of bacteria as biological factories. Mammals have also been genetically modified to produce much needed therapeutic proteins in their milk. This re-engineering of biological systems for new purposes has great potential in medicine. Nanotechnology is another strand of synthetic biology, where tiny, non-natural particles are used for biological purposes - for example, to deliver drugs to very specific sites within the cells of pathogens or tumours.

C. difficile

Antibiotic-resistant bacteria are a particular problem in hospitals and care homes for older people, where antibiotics are often needed and used. Clostridium difficile has been a high-profile example of antibiotic-resistant bacteria. - Bacterium in the guts of about 5% of the population - Produces toxins that damage the lining of the intestines, leading to diarrhoea, bleeding and even death - Not a problem for healthy person but when commonly-used antibiotics kill off much of the 'helpful' gut bacteria it survives, reproduces and takes hold rapidly.

MRSA

Antibiotic-resistant bacteria are a particular problem in hospitals and care homes for older people, where antibiotics are often needed and used. MRSA (methicillin-resistant Staphylococcus aureus) has been a high-profile example of antibiotic-resistant bacteria. - Bacterium carried by up to 30% of the population on their skin or in their nose - In the body it can cause boils, abscesses and potentially fatal septicaemia - Was treated effectively with methicillin, a penicillin-like antibiotic but mutation has produced methicillin-resistant strains

Selective toxicity

Antibiotics interfere with the metabolism of the bacteria without affecting the metabolism of the human cells - this is called selective toxicity.

Antibodies

Antibodies are Y shaped glycoproteins called immunoglobulins, which bind to a specific antigen on the pathogen or toxin that has triggered the immune response. There are millions of different antibodies, and there is a specific antibody for each antigen. Antibodies are made up of two identical long polypeptide chains called the heavy chains and two much shorter identical chains called the light chains. The chains are held together by disulfide bridges and there are also disulfide bridges within the polypeptide chains holding them in shape. Antibodies bind to antigens with a protein-based 'lock and key' mechanism similar to the complementarity between the active site of an enzyme and its substrate. The binding site is an area of 110 amino acids on both the heavy and the light chains, known as the variable region. It is a different shape on each antibody and gives the antibody its specificity. The rest of the antibody molecules is always the same, so it is called the constant region. When an antibody binds to an antigen, it forms a antigen-antibody complex.

HIV/AIDS

Caused by HIV (human immunodeficiency virus), which targets T helper cells in the immune system of the body. It gradually destroys the immune system so affected people are open to other infections, such as TB, as well as some types of cancer. HIV/AIDS can affect humans and some non-human primates. HIV is a retrovirus with RNA as its genetic material. It contains the enzyme reverse transcriptase, which transcribes the RNA to a single strand of DNA to produce a single strand of DNA in the host cell. The virus is passed from one person to another in bodily fluids, most commonly through unprotected sex, shared needles, contaminated blood products, and from mothers to their babies during pregnancy or breast feeding. There is as yet no vaccine or cure, but anti-retroviral drugs slow the progress of the disease to give many years of healthy life. Girls and women are at particularly high risk of HIV in many countries. Traditions practices such as female genital mutilation (FGM) increase the infection rate. This disease has massive social and economic consequences as well as the personal impact to each person infected.

Malaria

Caused by the protoctista Plasmodium and spread by the bites of infected Anopheles mosquitoes. The Plasmodium parasite has a complex life cycle with two hosts - mosquitoes and people. They reproduce inside the female mosquito. The female needs to take two blood meals to provide her with protein before she lays her eggs - and this is when Plasmodium is passed on to people. It invades the red blood cells, liver and even the brain. Around 200 million people are reported to have malaria each year and over 600,000 die. The disease recurs, making people weak and vulnerable to other infections. There is no vaccine against malaria and limited cures, but preventative measures can be very effective. The key is to control the vector. Anopheles mosquitoes can be destroyed by insecticides and by removing standing water where they breed. Simple measures such as mosquito nets, window and door screens and long sleeved clothing can prevent them biting people and spreading the disease.

Communicable diseases

Communicable diseases are caused by infective organisms known as pathogens. Each has particular characteristics that affect the way they are spread and the ways we can attempt to prevent or cure the diseases they cause. A communicable disease can be passed on from one organism to another. In animals they are most commonly spread from one individual of a species to another, but they can also be spread between species. Communicable diseases in plants are spread directly from plant to plant. Vectors, which carry pathogens from one organism to another, are involved in the spread of a number of important plant and animal diseases. Common vectors include water and insects.

What are the two modes of action pathogens take?

Damaging the host tissues directly Producing toxins which damage host tissues

What are the two main types of transmission of communicable diseases?

Direct transmission Indirect transmission

Artificial passive immunity

For certain potentially fatal diseases, antibodies are formed in one individual (often an animal), extracted and the injected into the bloodstream of another individual. This artificial passive immunity gives temporary immunity - it doesn't last long but it can be life saving. For example, rabies is a fatal disease that is treated with a series of injections that give artificial passive immunity.

Fungi

Fungi are eukaryotic organisms that are often multicellular, although the yeasts which cause human diseases such as thrush are single-celled. Fungi cannot photosynthesise and they digest their food extracellularly before absorbing nutrients. Many fungi are saprophytes which means they feed on dead and decaying matter. However some fungi are parasitic, feeding on living plants and animals. These are the pathogenic fungi which cause communicable diseases. Because fungal infections often affect the leaves of plants, they stop them photosynthesising and so can quickly kill the plant. When fungi reproduce they produce millions of tiny spores which can spread huge distances, this adaptation means they can spread rapidly and widely through crop plants. Fungal diseases of plants cause hardship and even starvation in many countries around the world.

Why is it difficult to develop a vaccine for HIV?

HIV, the human immunodeficiency virus that causes AIDS. It enters the macrophages and T helper cells, so it has disabled the immune system itself.

Direct transmission in animals

Here the pathogen is transferred directly from one individual to another by: Direct contact (contagious diseases): - Kissing or any other contact with bodily fluids of another person, e.g STDs - Direct skin-to-skin contact, e.g. athlete's foot - Microorganisms from faeces transmitted on hands, e.g. diarrhoeal diseases Inoculation: - Through a break in the skin, e.g. during sex (HIV/AIDS) - From an animal bite, e.g. Rabies - Through a puncture wound or through sharing needles, e.g. septicaemia Ingestion - Taking in contaminated food or drink, or transferring pathogens to the mouth from the hands, e.g. amoebic dysentery

Non-specific defences - getting rid of pathogens

If the pathogens get into the body, the next lines of defence are adaptations to prevent them growing or to destroy them.

Secondary immune response

If the same pathogen enters the body again, the immune system will produce a quicker, stronger immune response - the secondary response. Clonal selection happens faster. Memory B lymphocytes divide into plasma cells that produce the right antibody to the antigen. Memory T lymphocytes are activated and divide into the correct type of T lymphocytes to kill the cell carrying the antigen. The secondary response often gets rid of the pathogen before you begin to show any symptoms.

Blood clotting and wound repair

If you cut yourself, the skin is breached and pathogens can enter the body. The blood clots rapidly to seal the wound. When platelets come into contact with collagen in skin or the wall of the damaged blood vessel, they adhere and begin secreting several substances. The most important are: - Thromboplastin, an enzyme that triggers a cascade of reactions resulting in the formation of a blood clot (or thrombus) - Serotonin, which makes the smooth muscle in the walls of the blood vessels contract, so they narrow and reduce the supply of blood to the area The clot dries out, forming a hard, tough scab that keeps pathogen out. This is the first stage of wound repair. Epidermal cells below the scab start to grow, sealing the wound permanently, while damaged blood vessels regrow. Collagen fibres are deposited to give the new tissues strength. Once the new epidermis reaches normal thickness, the scab sloughs off and the wound is healed.

Artificial active immunity

In artificial active immunity the immune system of the body is stimulated to make its own antibodies to a safe form of an antigen (a vaccine), which is injected into the bloodstream (vaccination). The antigen is not usually the normal live pathogen, as this could cause the disease and have fatal results.

Cell-mediated immunity

In cell-mediated immunity, T lymphocytes respond to the cells of an organism that have been changed in some way, for example by a virus infection, by antigen processing or by mutation (for example cancer cells) and to cells from transplanted tissue. The cell-mediated response is particularly important against viruses and early cancers. 1) In the non-specific defence system, macrophages engulf and digest pathogens in phagocytosis. They process the antigens from the surface of the pathogen to form antigen-presenting cells (APCs). 2) The receptors on some of the T helper cells fit the antigens. These T helper cells become activated and produce interleukins, which stimulate more T cells to divide rapidly by mitosis. They form clones of identical activated T helper cells that all carry the right antigen to bind to a particular pathogen. 3) The cloned T cells may: - Develop into T memory cells, which give a rapid response if this pathogen invades the body again - Produce interleukins that stimulate phagocytosis - Produce interleukins that stimulate B cells to divide - Stimulate the development of a clone of T killer cells that are specific for the presented antigen and then destroy infected cells.

Humoral immunity

In humoral immunity the body responds to antigens found outside the cells, for example bacteria and fungi, and to APCs. The humoral immune system produces antibodies that are soluble in the blood and tissue fluid and are not attached to cells. B lymphocytes have antibodies on their cell-surface membrane (IgM) and there are millions of different types of B lymphocytes, each with different antibodies. When a pathogen enters the body it will carry specific antigens, or produce toxins that act as antigens. A B cell with the complementary antibodies will bind to the antigens on the pathogen, or to the free antigens. The B cell engulfs and processes the antigens to become an APC. 1) Activated T helper cells bind to the B cell APC. This is clonal selection - the point at which the B cell with the correct antibody to overcome a particular antigen is selected for cloning 2) Interleukins produced by the activated T helper cells activate the B cells 3) The activated B cell divides by mitosis to give clones of plasma cells and B memory cells. This is clonal expansion. 4) Cloned plasma cells produce antibodies that fit the antigens on the surface of the pathogen, bind to the antigen and disable them, or act as opsonins or agglutinins. This is the primary immune response and it can take days or even weeks to become fully effective against a particular pathogen. This is why we get ill, the symptoms are a result of the way our body reacts when the pathogens are dividing freely, before the primary immune response is fully operational. 5) Some cloned B cells develop into B memory cells. If the body is infected by the same pathogen again, the B memory cells divide rapidly to form plasma cell clones. These produce the right antibody and wipe out the pathogen very quickly, before it can cause the symptoms of disease. This is the secondary immune response.

Why is it difficult to develop a vaccine for malaria?

Malaria - Plasmodium, the protoctist that causes malaria. It is very evasive - it spends time inside the erythrocytes so it is protected by self antigens from the immune system, and within an infected individual its antigens reshuffle.

Chemical defences in plants

Many plants produce powerful chemicals that either repel the insect vectors of disease or kill invading pathogens. Examples of plant defensive chemicals include: - Insect repellents - for example, pine resin - Insecticides - for example, caffeine which toxic to insects and fungi - Antibacterial compounds including antibiotics - for example, antibacterial gossypol produced by cotton - Antifungal compounds - for example, phenols, antifungals made in many different plants; chitinases - enzymes that break down the chitin in fungal cell walls - Anti-oomycetes - for example, glucanases - enzymes made by some plants that break down glucans - General toxins - some plants make chemicals that can be broken down to form cyanide compounds when the plant cell is attacked. Cyanide is toxic to most living things.

What are the two lines of defence in animals?

Non-specific defences Specific immune response

Damaging the host tissues directly

Many types of pathogen damage the tissues of their host organism. It is in this damage, combined with the way in which the body of the host responds to the damage, that causes the symptoms of disease. Different types of pathogens attack and damage the host tissues in different ways. Viruses take over the cell metabolism. The viral genetic material gets into the host cell and is inserted into the host DNA. The virus then uses the host cell to make new viruses which then burst out of the cell, destroying it and then spread to infect other cells. Some protoctista also take over cells and break them open as the new generation emerge, but they do not take over the genetic material of the cell. They simply digest and use the cell contents as they reproduce. Proctists which cause malaria are an example of this. Fungi digest living cells and destroy them. This combined with the response of the body to the damage caused by the fungus gives the symptoms of disease.

Medicines

Medicines can be used to treat communicable and non-communicable diseases. Medicines can be used to treat symptoms and cure them, making people feel better. Common medicines include painkillers, anti-inflammatories and anti-acid medicines (which reduce indigestion). Medicines that cure people include chemotherapy against some cancers, antibiotics that kill bacteria, and antifungals that kill fungal pathogens.

Maintaining immunity

Memory B and T lymphocytes only have a limited lifespan. This means that someone who is immune to a particular pathogen won't always stay immune forever - once all of the memory B and T lymphocytes have died, the person may be susceptible to attack by the pathogen again. Immunity can be maintained by being continually exposed to the pathogen, so you continue to make more and more memory B and T lymphocytes.

Pathogens

Microbes that cause disease

What measures help reduce antibiotic-resistant infections in the long term?

Minimising the use of antibiotics and ensuring that every course of antibiotics is completed to reduce the risk of resistant individuals surviving and developing into a resistant strain population Good hygiene in hospitals, care homes and in general - this has a major impact on the spread of all infections, including antibiotic-resistant strains

Producing toxins which damage host tissues

Most bacteria produce toxins that poison or damage the host cells in some way, causing disease. Some bacterial toxins damage the host cells by breaking down the cell membranes, some damage or inactivate enzymes and some interfere with the host cell genetic material so the cells cannot divide. These toxins are a by-product of the normal functioning of the bacteria. Some fungi produce toxins which affect the host cells and cause disease.

Opsonins

Opsonins are chemicals that bind to pathogens and 'tag' them so they can be more easily recognised by phagocytes. Phagocytes have receptors on their cell membranes that bind to common opsonins, and the phagocyte then engulfs the pathogen. There are a number of different opsonins, but antibodies such as immunoglobulin G (IgG) and immunoglobulin M (IgM) have the strongest effect.

What types of pathogens are there?

Pathogens include: - bacteria - viruses - fungi - protoctista.

Penicillin

Penicillin was the first widely used, effective, safe antibiotic capable of curing bacterial diseases. It comes from a mould. Penicillium chrysogenum, famously discovered by Alexander Fleming in 1928, when he found it growing on his Staphylococuss spp. cultures. Fleming saw what the mould did to his bacteria but could not extract enough to test its potential. It needed Florey and Chain to develop an industrial process for making the new drug, which has since saved millions of lives around the world.

Pharmacogenetics

Personalised medicine - a combination of drugs that work with your individual combination of genetics and disease - is the direction in which medicine is going. The human genome can be analysed relatively rapidly and cheaply, giving a growing understand of the genetic bases of many diseases. The science of interweaving of drug actions with personal genetic material is known as pharmacogenomics. We already know that genotypes and drugs interact. In the future, treatment where clinicians looks at the genome of their patients and the genome of the invading pathogen before deciding how to treat them will become increasingly common.

Phagocytes

Phagocytes are specialised white cells that engulf and destroy pathogens. There are two main types of phagocytes - neutrophils and macrophages. Phagocytes build up at the site of an infection and attack pathogens. Sometimes you can see pus in a spot, cut or wound. Pus consists of dead neutrophils and pathogens.

Cytokines

Phagocytes that have engulfed a pathogen produce chemicals called cytokines. Cytokines act as cell-signalling molecules, informing other phagocytes that the body is under attack and stimulating them to move to the site of infection or inflammation. Cytokines can also increase body temperature and stimulate the specific immune system.

Main types of B lymphocytes

Plasma cells - these produce antibodies to a particular antigen and release them into their circulation. An active plasma cell only lives for a few days but produces around 2000 antibodies per second while it is alive and active B effector cells - these divide to form the plasma cell clones. B memory cells - these live for a very long time and provide their immunological memory. They are programmed to remember a specific antigen and enable the body to make a very rapid response when a pathogen carrying that antigen is encountered again.

Potato blight (tomato blight, late blight)

Potato blight (tomato blight, late blight) - caused by the fungus-like protoctist oomycete Phytophthora infestans. The hyphae penetrate host cells, destroying leaves, tubers and fruit, causing millions of pounds worth of crop damage each year. There is no cure but resistant strains, careful management and chemical treatments can reduce infection risk.

Indirect transmission in plants

Soil Contamination Infected plants often leave pathogens (bacteria or viruses) or reproductive spores from protoctista or fungi in the soil. These can infect the next crop. Examples are black sigatoka spores and TMV. Some pathogens (often as spores) can survive the composting process so the infection cycle can be completed when contaminated compost is used. Vectors - Wind - bacteria, viruses and fungal or oomycete spores may be carried on the wind. E.g. black sigatoka blown between Caribbean Islands - Water - spores swim to the surface film of water on leaves; raindrop splashes carry pathogens and spores, etc. Examples are spores of potato blight which swim over films of water on the leaves - Animals - insects and birds carry pathogens and spores from one plant to another as they feed. Insects such as aphids inoculate pathogens directly into plant tissues. - Humans - pathogens and spores are transmitted by hands, clothing, fomites, farming practices and by transporting plants and crops around the world. For example, TMV survives for years in tobacco products

Transmission between animals and humans

Some communicable diseases can be passed from animals to people, for example the bird flu strain H1N1. Minimising close contact with animals and washing hands thoroughly following any such contact with animals can reduce infection rates. People can also act as vectors of some animal diseases, sometimes with fatal results, e.g. foot-and-mouth disease.

Artificial immunity

Some diseases kill people before their immune system makes the antibodies they need. Medical science can give us immunity to some of these life-threatening diseases without any contact with the live pathogens.

Natural immunity

Some forms of immunity occur naturally in the body: When you meet a pathogen for the first time, your immune system is activated and antibodies are formed, which results in the destruction of the antigen. The immune system produces T and B memory cells so if you meet a pathogen for a second time, your immune system recognises the antigens and can immediately destroy the pathogen, before it causes disease symptoms. This is known as natural active immunity. It is known as active because the body has itself acted to produce antibodies and/or memory cells. The immune system of a new-born baby is not mature and it cannot make antibodies for the first couple of months. A system has been evolved to protect the baby for those first few months of life. Some antibodies cross the placenta from the mother to her fetus while the baby is in the uterus, so it has some immunity to disease at birth. The first milk a mammalian mother makes is called colostrum, which is very high in antibodies. The infant gut allows these glycoproteins to pass into the bloodstream without being digested. So within a few days of birth, a breastfed baby will have the same level of antibody protection against disease as the mother. This is natural passive immunity and it lasts until the immune system of the baby begins to make its own antibodies. The antibodies the baby receives from the mother are likely to be relevant to pathogens in its environment, where the mother acquired them.

Autoimmune diseases

Sometimes the immune system stops recognising 'self' cells and starts to attack healthy body tissue. This is termed an autoimmune disease. Scientists still do not understand fully why this happens. There appears to be a genetic tendency in some families, sometimes the immune system responds abnormally to a mild pathogen or normal body microorganisms and is some cases the T regulator cells do not work effectively. There are around 80 different autoimmune diseases that can cause inflammation or the complete breakdown and destruction of healthy tissue. Immunosuppressant drugs, which prevent the immune system working, may be used as treatments but they deprive the body of its natural defences against communicable diseases.

Main types of T lymphocytes

T helper cells - these have CD4 receptors on their cell-surface membranes, which bind to the surface antigens on APCs. They produce interleukins, which are a type of cytokine (cell-signalling molecule). The interleukins made by the T helper cells stimulate the activity of B cells, which increases antibody production, stimulate production of other types of T cells and attracts and stimulates macrophages to ingest pathogens with antigen-antibody complexes T killer cells - these destroy the pathogen carrying the antigen. They produce a chemical called perforin, which kills the pathogen by making holes in the cell membrane so it is freely permeable. T memory cells - these live for a long time and are part of the immunological memory. If they meet an antigen a second time, they divide rapidly to form a huge number of clones of T killer cells that destroy the pathogen T regulator cells - these cells suppress the immune system, acting to control and regulate it. They stop the immune response once a pathogen has been eliminated, and make sure the body recognises self antigens and does not set up an autoimmune response. Interleukins are important in this control.

How do antibodies defend the body?

The antibody of the antigen-antibody complex acts as an opsonin so the complex is easily engulfed and digested by phagocytes Most pathogens can no longer effectively invade the host cells once they are part of an antigen-antibody complex. Antibodies act as agglutinins causing pathogens carrying antigen-antibody complexes to clump together. This helps prevent them spreading through the body and makes it easier for phagocytes to engulf a number of pathogens at the same time Antibodies can act as anti-toxins, binding to the toxins produced by pathogens and making them harmless.

Non-specific defences - keeping pathogens out

The body has a number of barriers to the entry of pathogens: The skin covers the body and prevents the entry of pathogens. It has a skin flora of healthy microorganisms that outcompete pathogens for space on the body surface. The skin also produces sebum, an oily substance that inhibits growth of pathogen Many of the body tracts, including the airways of the gas exchange system, are lined by mucous membranes that secrete sticky mucus. This traps microorganisms and contains lysozymes, which destroy bacterial and fungal cell walls. Mucus also contains phagocytes, which remove remaining pathogens. Lysozymes in tears and urine, and the acid in the stomach, also help to prevent pathogens getting into our bodies We also have expulsive reflexes. Coughs and sneezes eject pathogen-laden mucus from the gas exchange system, while vomiting and diarrhoea expel the contents of the gut along with any infective pathogens.

Inflammatory response

The inflammatory response is a localised response to pathogens (or damage or irritants) resulting in inflammation at the site of a wound. Inflammation is characterised by pain, heat, redness and swelling of tissue. Mast cells are activated in damaged tissue and release chemicals called histamines and cytokines. - Histamines makes the blood vessels dilate, causing localised heat and redness. The raised temperature helps prevent pathogens reproducing - Histamines make blood vessel walls more leaky so blood plasma is forced out, once forced out of the blood it is known as tissue fluid. Tissue fluid causes swelling (oedema) and pain - Cytokines attract white blood cells (phagocytes) to the site. They dispose of pathogens by phagocytosis. If an infection is widespread, the inflammatory response can cause a whole-body rash.

Sources of medicines

The medicines we use today come from a wide range of sources. Scientists design drugs using complex computer programmes. They can build up 3D models of key molecules in the body and of pathogens and their antigen systems. This allows models of potential drug molecules to be built up which are targeted at particular areas of a pathogen. Computers are also used to search through enormous libraries of chemicals, to isolate any with a potentially useful action against a specific group of feature of a pathogen, or against the mutated cells in a cancer. Analysis of the genomes of pathogens and genes which have been linked to cancer enable scientists to target their novel drugs to attack any vulnerabilities. However, many of the drugs most commonly used in medicine are still either derived from, or based on, bioactive compounds discovered in plants, microorganisms or other forms of life.

Factors affecting the transmission of communicable diseases in animals

The probability of catching a communicable disease is increased by a number of factors: - Overcrowded living and working conditions Poor nutrition - A compromised immune system, including (in humans) having HIV/AID or needing immunosuppressant drugs after transplant surgery - (In humans) poor disposal of water, providing breeding sites for vectors - Climate change - this can introduce new vectors and new diseases - Culture and infrastructure - in many countries traditional medical practises can increase transmission - Socioeconomic factors - for example, a lack of trained health workers and insufficient public warning when there is an outbreak of disease

Protoctista (protista)

The protoctista are a group of eukaryotic organisms with a wide variety of feeding methods. They include single-celled organisms and cells grouped into colonies. A small percentage of protoctista act as pathogens, causing devastating communicable diseases in both animals and plants. The protists which cause disease are parasitic - they use people or animals as their host organism. Pathogenic protists may need a vector to transfer them to their hosts - malaria and sleeping sickness are examples - or they may enter the body directly through polluted water - amoebic dysentery is an example of this.

The specific immune system

The specific immune system (also known as active or acquired immunity) is slower than the non-specific responses - it can take up to 14 days to respond effectively to a pathogen invasion. However, the immune memory cells mean it reacts very quickly to a second invasion by the same pathogen.

Lymphocytes and the immune response

The specific immune system is based on white blood cells called lymphocytes. B lymphocytes mature in the bone marrow, while T lymphocytes mature in the thymus gland.

Bacteriophages

There are even viruses that attack bacteria, known as bacteriophages. They take over the bacterial cells and use them to replicate, destroying the bacteria at the same time. People now use bacteriophages both to identify and treat some diseases, and they are very important in scientific research.

The development of antibiotic resistance

There is an evolutionary race between scientists and bacteria. An antibiotic works because a bacterium has a binding site for the drug, and a metabolic pathway that is affected by the drug. If a random mutation during bacterial reproduction produces a bacterium that is not affected by the antibiotic, that is the one which is best fitted to survive and reproduce, passing on the antibiotic resistance mutation to the daughter cells. Bacteria reproduce very quickly, so once a mutation occurs it does not take long to grow a big population of antibiotic-resistant bacteria. In a few decades we have reached a stage where increasing numbers of bacterial pathogens are resistant to most or all of our antibodies.

Direct transmission in plants

This involves direct contact of a healthy plant with any part of a diseased plant. Examples are ring rot, tobacco mosaic virus, tomato and potato blight, and black sigatoka.

Indirect transmission in animals

This is where the pathogen travels from one individual to another indirectly: Fomites - Inanimate objects such as socks can transfer pathogens e.g Athletes Foot Droplet Infection (Inhalation) - Minute droplets of saliva and mucus are expelled from your mouth as you talk, cough or sneeze. If these droplets contain pathogens, when healthy individuals breath the droplets in they may become infected e.g TB Vectors - A vector transmits communicable pathogens from one host to another and are often but not always animals, e.g mosquitoes transmit malaria - Water can also act as a vector of disease, e.g. diarrhoeal diseases

How do vaccines prevent epidemics?

Vaccines are used to give long-term immunity to many diseases. However, they are also used to help prevent epidemics. An epidemic is when a communicable disease spreads rapidly to a lot of people at a local or national level. A pandemic is when the same disease spreads rapidly across a number of countries and continents. At the beginning of an epidemic, mass vaccinations can prevent the spread of the pathogen into the wider population. When vaccines are being deployed to prevent epidemics, they often have to be changed regularly to remain effective. When a significant number of people in the population have been vaccinated, this gives protection to those who do not have immunity. This is known as herd immunity, as there is minimal immunity for an outbreak to occur.

What do vaccines usually contain?

Vaccines may contain: Killed or inactivated bacteria and viruses (e.g. whooping cough) Attenuated (weakened) strains of live bacteria or viruses (e.g. rubella) Toxin molecules that have been altered and detoxified (e.g. tetanus) Isolated antigens extracted from the pathogen (e.g the influenza vaccine) Genetically engineered antigens (e.g. the hepatitis B vaccine)

Viruses

Viruses are non-living infectious agents. At 0.02-0.3 micrometres in diameter, they are around 50 times smaller in length than the average bacterium. The basic structure of a virus is some genetic material (DNA or RNA) surrounded by a protein. Viruses invade living cells, where the genetic material of the virus takes over the biochemistry of the host cell to make more viruses. Viruses reproduce rapidly and evolve by developing adaptations to their host, which makes them very successful pathogens. All naturally occurring viruses are pathogens. They cause disease in every other type of organism. Medical scientists consider viruses to be the ultimate parasites.

Primary immune response

When a pathogen enters the body for the first time, the antigens on its surface activate the immune system. This is called the primary response. The primary response is slow because there aren't many B lymphocytes that can make the antibody needed to bind to the pathogen. Eventually the body will produce enough of the right antibody to overcome the infection. Meanwhile the infected person will show symptoms of the disease. After being exposed to an antigen, both T and B lymphocytes produce memory cells. These memory cells remain in the body for a long time. Memory T lymphocytes remember the specific antigen and will recognise it a second time around. Memory B lymphocytes record the specific antibodies needed to bind to the antigen. The person is now immune - their immune system has the ability to respond quickly to a second infection.

Identifying pathogens

When an outbreak of a disease occurs in plants of animals, the key to successful control or cure is to identify the pathogens involved. Our ability to do this has increased along with our understanding of the causes of disease and developments in technology: - Traditionally pathogens were cultured in the laboratory and identified using a microscope - Monoclonal antibodies can be used now to identify pathogenic organisms in both plants and animals - DNA sequencing technology means pathogens can be identified precisely, down to a single mutation

Physical defences in plants

When plants are attacked by pathogens they rapidly set up extra mechanical defences. They produce high levels of a polysaccharide called callose, which contains β-1,3 linkages and β-1,6 linkages between the glucose monomers. Scientists still do not fully understand the roles played callose in the defence mechanisms of the plant but current research suggests that: - Within minutes of the initial attack, callose is synthesised and deposited between the cell walls and the cell membrane in cells next to the infected cells. These callose papillae act as barriers, preventing the pathogens entering the plant cells around the site of infection - Large amounts of callose continue to be deposited in cell walls after the initial infection. Lignin is added, making the mechanical barrier to invasion even thicker and stronger - Callose blocks sieve plates in the phloem, sealing off the infected part and preventing the spread of pathogens. - Callose is deposited in the plasmodesmata between infected cells and their neighbours, sealing them off from the healthy cells and helping to prevent the pathogen spreading

Counting blood cells

You have learnt how to examine microscope slides and draw cells, as well as count the number of cells in a given area of slide. Both of these skills are important when looking at blood smears, made by spreading a single drop of blood very thinly across a slide. They are often stained to show up the nuclei of the lymphocytes, making them easier to identify. Identifying the numbers of different types of lymphocytes in a blood smear indicates if a non-specific or specific immune response is taking place.