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Chapter 10 Pathogens and immunity

By Aditi Atmasidha Chapter 10 Pathogens and immunity

Mary Jones And Geoff Jones Cambridge IGCSE Biology Course book Third edition 1

What is a pathogen?A pathogen is a micro-organism that can cause harm to your body. A disease is caused when a pathogen enters our body and breeds there.

Types of pathogens There are many types of pathogens. Few of them are listed below with their properties:

Group to which pathogen belongs Examples of diseases which they cause VirusesInfluenza, common cold, poliomyelitis, AIDSBacteria Cholera, syphilis, whooping cough, tuberculosis, tetanus Protoctists Malaria, amoebic dysentry Fungi Athletes foot, ringworm

Types of pathogens

How do pathogens enter the body? Pathogens enter the human body in two ways, either by direct means or indirect means. Direct contact: Direct contact means that the disease-causing microbe is passed from one person to another when their bodies touch in some way.Indirect contact: Indirect contact happens when microorganisms are carried to a person in some way, instead of by actual body to body contact.Indirect transmission takes place in three ways: through respiratory passage, food & water, and through vector.

Indirect transmission Vector transmissioninvolves an animal such as an insect. For example, malaria is transmitted by a female anopheles mosquitoes, dysentery by houseflies and plague by fleas. Food transmission Eating raw or undercooked food, or drinking water contaminated by sewage means you take large numbers of microorganisms straight into your gut eg,Salmonella.Water transmission contaminated by sewage means you take large numbers of microorganisms straight into your gut eg cholera and amoebic dysentery. Anywhere without proper sewage treatment and disposal is at risk of these diseases. *contaminated : make impure by exposure to or addition of a poisonous or polluting substance.

Transmissible Disease Therefore a Transmissible Disease is a disease in which the pathogen can be passed from one host to another.

Body defences The body has several defences against pathogens so we do not fall ill with the diseases they cause.The skinThe skin covers the whole body. It protects the body from physical damage, microbe infection and dehydration. Its dry, dead outer cells are difficult for microbes to penetrate, and the sebaceous glands produce oils which help kill microbes.

Body defences Blood clottingIf microorganisms get into the body through a cut in the skin, the most important thing to do is close the wound quickly so that no more microorganisms can enter. A scab does just that. The blood contains tiny structures called platelets, and a protein called fibrin. A scab is basically platelets stuck in a fibrin mesh.Mucous membranesThe respiratory system is protected in several ways. Nasal hairs keep out dust and larger microorganisms. Sticky mucus traps dust and microbes, which are then carried away by cilia - tiny hairs on the cells that line the respiratory system.

Body defences Stomach acidHydrochloric acid in the stomach kills harmful microorganisms that might be in the food or drink that we swallow.

Food hygiene Keep your own bacteria and viruses away from the food. Always wash your hands before touching or eating food or putting your hands into your mouth for any reason. Keep your hair out of food. Never cough or sneeze over food. Keep animals away from the foods. For example, if we leave food uncovered and allow houseflies to settle on them, then they bring with them bacteria. As they continue feeding they spit their saliva on the food and contaminate it. Do not keep foods at room temperature for long time or the bacteria start breeding in it. Keep raw meat away from foods as it contains bacteria.

Waste disposal

Waste disposal Waste is separated into biodegradable and non- biodegradable wastes.Biodegradable waste includes, waste food, animal organs and remains of plants and animals. This waste is collected and taken to landfill sites, and is covered with soil. The covered soil compacts the waste. Due to the pressure above the soil, the water is squeezed out from the waste, this water is pumped out and treated. The waste is then decomposed due to the action of bacteria present in the soil. This releases methane gas and if trapped for a long time can result into explosion hence it is taken out through pipes and is used as fuel.

Sewage treatment Sewage is waste liquid that comes from houses, industries, villages, towns and cities. It includes mostly water. It also includes urine, feces, toilet paper, detergents, etc. Sewage is treated to remove pathogenic organisms and most of the nutrients. When it is treated, water can recycled.

How immunity developsDuring the bodys first encounter with a pathogen there will be few lymphocytes with specific receptors

It takes time to divide to form clones, B lymphocytes to secrete antibodies, T lymphocyte production

If the same pathogen invades again persisting memory cells can give a faster, more effective response

Secondary response happens more quickly, and produces many more antibodiesThis is ACTIVE IMMUNITY

An infection is an example of acquiring natural immunity. It is called ACTIVE as your body needs to work to produce the necessary antibodies

When a mother breast feeds her baby she passes antibodies to it. This is a way of acquiring PASSIVE immunity as it is a way of gaining antibodies without the immune system having to produce them. The thick, yellowish milk (colostrum) that is produced for the first few days after birth is particularly rich in antibodies. Natural immunity: active and passive

Artificial immunity: active and passiveAn alternative to natural immunity developing is to give vaccinations (artificial immunity)Antigen is injected into the body. This may be in the form of an inactivated bacterial toxin or attenuated (not harmful) virus which would promote ACTIVE immunity; or the injection of antibodies or antitoxins which would promote PASSIVE immunity (eg Clostridium tetani)

Passive immunityPassive immunity doesnt last as long as active immunity (only weeks or months):No lymphocytes are stimulated to clone themselvesNo memory cells have been madeThis type of immunity can only last as long as the antibodies/toxins last in the blood


Antigen binds with variable region (specific)Have disulphide bonds between different chains of the antibodyAlso known as immunoglobulins shortened to Ig such as IgA or IgEDifferent immunoglobulins respond to antigens in different ways

Neutralisation: bind with bacterial toxins

Agglutination: make pathogen clump together

Phagocytosis: often follows agglutinationAntibodies

How lymphocytes react to pathogens?