Infection and response

• Pathogens are microorganisms that cause infectious disease. Pathogens may be viruses, bacteria, protists or fungi. They may infect plants or animals and can be spread by direct contact, by water or by air. Bacteria and viruses may reproduce rapidly inside the body. Bacteria may produce poisons (toxins) that damage tissues and make us feel ill. Viruses live and reproduce inside cells, causing cell damage.
• Measles is a viral disease showing symptoms of fever and a red skin rash. Measles is a serious illness that can be fatal if complications arise. For this reason most young children are vaccinated against measles. The measles virus is spread by inhalation of droplets from sneezes and coughs.
• Antibodies are proteins that bind to antigens on pathogens
• Inflammation occurs when an area becomes red, swollen and painful due to increased blood flow. It helps prevent further invasion by pathogens because the heat kills some pathogens and the extra fluid washes away any remaining pathogens. White blood cells move into the damaged tissue where they engulf and digest pathogens.
• Antibodies are proteins produced by plasma cells. Antibodies bind to antigens on the surface of pathogens. This makes them easier for phagocytic cells to engulf and destroy. Some antibodies also trigger other defences such as inflammation and fever.
• The immune system is made up of specialised white blood cells called lymphocytes which recognise pathogens as foreign invaders. Lymphocytes have receptors on their surface that fit to specific antigens found only on the surfaces of pathogens. When a lymphocyte meets its target antigen it divides many times producing clones of identical daughter cells. These clone cells all attack the same type of pathogen. The more clones produced, the faster the infection will be cleared from the body.
• HIV initially causes a flu-like illness. Unless successfully controlled with antiretroviral drugs the virus attacks the body’s immune cells. Late stage HIV infection, or AIDS, occurs when the body's immune system becomes so badly damaged it can no longer deal with other infections or cancers. HIV is spread by sexual contact or exchange of body fluids such as blood which occurs when drug users share needles.
• 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.
• Viruses reproduce inside living host cells. Virus particles enter the cell and take over control of the cell machinery. New copies of the virus are then assembled within the cell. Eventually the new virus particles burst out of the cell killing it. The newly formed virus particles go on to infect other cells.
• Fever is caused by chemicals called pyrogens released by immune system cells. Pyrogens increase the temperature of the body which makes it harder for many pathogens to survive.
• A vaccine contains harmless versions of the pathogen or parts of the pathogen. Vaccines stimulate the production of antibodies without causing disease. Once vaccinated, if someone comes into contact with the real pathogen, their immune system responds quickly to produce large numbers of antibodies to fight off the infection.
• Fungi cause skin infections like athlete’s foot and ringworm. Fungal spores enter the body through cuts in the skin. Yeast fungi live inside our bodies and can cause thrush.
• Bacteria reproduce rapidly through binary fission. They can survive outside the human body for long periods of time. Bacterial diseases include cholera, tuberculosis and food poisoning.
• Phagocytosis is the process by which certain types of white blood cells engulf and digest pathogens. Phagocytic cells include neutrophils and macrophages.