Ocr A Level Biology: Communicable Diseases and Immune System

Communicable Diseases, Disease Prevention & the Immune System

Contents

• Common Pathogens & Communicable Diseases
• Transmission of Communicable Pathogens
• Plant Defences Against Pathogens
• Non-specific Immune Responses
• Phagocytes
• Blood Cells
• The T Lymphocyte Response
• The B Lymphocyte Response
• Primary & Secondary Immune Responses
• Antibodies
• Opsonins, Agglutinins & Anti-toxins
• Types of Immunity
• Autoimmune Diseases
• Principles of Vaccination
• Sources of Medicine

Antibiotics

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Common Pathogens & Communicable Diseases

Common Pathogens & Communicable Diseases

A disease is an illness or disorder of the body or mind that leads to poor health Each disease is associated with a set of signs and symptoms

Communicable/infectious diseases are caused by pathogens and are transmissible (can be spread between individuals within a population)

Both plants and animals can be affected by pathogens

Infectious & Non-Infectious Diseases Table

Definition Examples

Infectious disease

Disease caused by a pathogen that passes from infected individuals to uninfected individuals

Cholera, HIV/AIDS, malaria, tuberculosis

Also known as communicable disease

Non- infectious disease

Long-term diseases that are not caused by pathogens,

Lung cancer, cardiovascular disease, depression, cystic fibrosis, vitamin deficiencies

• Many microorganisms are pathogens including:
• Bacteria Viruses
• Fungi Protoctists

To control disease, it is very important to know what pathogen is causing it

Bacteria

Bacteria are a diverse range of prokaryotic organisms Some bacteria are non-pathogenic (they do not cause any disease or damage) while others are pathogenic

Pathogenic bacteria do not always infect the hosts of cells, they can remain within body cavities or spaces

• M. tuberculosis causes tuberculosis (TB) in humans

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The bacteria infect the lungs, causing a chronic cough and bloody mucus It is a disease often associated with poor hygiene and sanitation

• M. bovine in cows can also transmit to humans to cause TB Ring rot diseases in potato plants are caused by bacterial pathogens The bacteria infect the vascular tissue and prevent the transport of water, causing the plant to wilt and die
• The infection spreads into the potato tubers where the vascular tissue is arranged in a ring, producing the characteristic black ring of rot

Viruses

Viruses do not have a cellular structure This means they can't respire, produce ATP, replicate genetic material or synthesise protein

They infect host cells and hijack their machinery to replicate their own genetic material and proteins

The first virus ever discovered was the Tobacco Mosaic Virus (TMV) TMV infects several plant species It causes a distinct yellowing of the leaves which produces a mosaic pattern

• Three different influenza viruses infect humans to cause the flu Influenza A, influenza B and influenza C infect the cells that line the airways They cause a high temperature, body aches and fatigue Influenza A is the virus that causes the most cases of flu globally It has a capsid that surrounds 8 single-stranded molecules of RNA
• The human immunodeficiency virus (HIV) infects specific cells of the immune system It is an enveloped retrovirus The viral enzyme reverse transcriptase produces single-stranded DNA from its viral RNA
• DNA polymerase synthesises double-stranded DNA from this single-stranded DNA The double-stranded DNA is inserted into the host DNA and can remain inactive for many years Once activated the DNA provirus is used to synthesise new viruses

Protoctista

Protists are unicellular eukaryotes Plasmodium falciparum is a protist that causes severe forms of malaria in humans The parasite is spread by mosquitoes

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• Infected individuals experience fever, chills and fatigue
• P. infestans causes the infamous potato blight
• The pathogen is unusual as it has some fungal characteristics It is transmitted via spores The first signs of potato blight are small, dark brown marks on the leaves which quickly increase in size and number The protist destroys potato and tomato crops leaving them completely inedible

Fungi

• Fungi have a similar structure to plants Their eukaryotic cells have cell walls and large central vacuoles However, instead of being made of separate cells, their bodies consist of filaments known as hyphae These hyphae form a network and spread throughout a host/soil Fungal diseases are much more common in plants than animals
• Athletes foot is a fungal disease that exist on the surface of the skin Fungal diseases in plants tend to be much more serious and can threaten entire crops Black Sigatoka is a fungal disease in bananas . It spreads through the leaves of the plant, reducing its ability to photosynthesise The lack of photosynthesis causes parts of the leaf to die; producing black streaks Eventually, the whole leaf dies

Common Pathogens and Related Communicable Diseases in Humans Table

Pathogen type Disease Method of transmission

Bacterium Tuberculosis Airborne droplets

Virus HIV/AIDS Body fluids Influenza Airborne droplets

Protoctist Malaria Female mosquitoes

Fungus Athlete's foot Skin-to-skin contact with an infected individual Contact with an item of clothing

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Common Pathogens and Related Communicable Diseases in Plants Table

Pathogen type Disease Plant host Method of transmission

Bacterium Ring rot Potato, tomato An infected tuber can lead to the growth of infected new plants Contaminated soil, water and equipment

Virus Tobacco mosaic Tobacco Leaf-to-leaf contact between plants Humans touching different plants

Protoctist Late blight Potato, tomato Spores are carried by wind from plant to plant

Fungus Black sigakota Bananas Leaf-to-leaf contact Spread of spores by humans or within infected plant matter

Examiner Tips and Tricks

These notes contain details of the binomial names of pathogens in order to provide a broad coverage of information about the diseases, however you are not required to memorise the binomial names for your exams.

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Transmission of Communicable Pathogens

Transmission of Communicable Pathogens

Disease Transmission

In order for a population of pathogens to survive, they must be able to successfully transfer from host to host

If pathogens are unable to find new hosts then they will go extinct

Disease transmission is defined as the transfer of pathogens from an infected host to an uninfected host

Transmission can be very risky for pathogens

During the infective stages, pathogens produce a large number of individuals to increase the likelihood that some will find a new host and survive

Transmission Through Contact Between Individuals

Some pathogens are transferred through physical contact between individuals

If the leaves of plants infected with Tobacco Mosaic Virus (TMV) touch the leaves of another uninfected plant, particles of the virus are transmitted

Sometimes individuals being within close proximity to each other is sufficient for transmission

The influenza viruses are spread in the air via tiny droplets of water. An infected individual breathes out droplets containing the virus and they are breathed in by an uninfected individual

Spores can also be involved in the transmission of pathogens

Spores are very small reproductive structures that are released into the environment. They are dispersed via wind or water

Once they reach a food source (host) they begin growing

Depending on the organism, spores can be produced via mitosis or meiosis so they can be haploid or diploid

P. infestans which causes potato blight produces specialised spores called sporangia. These structures are adapted for wind dispersal

Transmission of HIV/AIDS

Human Immunodeficiency Virus is a retrovirus

The HIV virus is not transmitted by a vector (unlike in malaria)

The virus is unable to survive outside of the human body

HIV is spread by intimate human contact and can only be transmitted by direct exchange of body fluids

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• This means HIV can be transmitted in the following ways: sexual intercourse
• blood donation
• sharing of needles used by intravenous drug users from mother to child across the placenta mixing of blood between mother and child during birth from mother to child through breast milk

Transmission of Tuberculosis (TB)

• When infected people with the active form of the disease cough or sneeze, the Mycobacterium tuberculosis bacteria enter the air in tiny droplets of liquid TB is transmitted when uninfected people then inhale these droplets TB, therefore, spreads more quickly among people living in overcrowded conditions
• The form of TB caused by Mycobacterium bovis occurs in cattle but is spread to humans through contaminated meat and unpasteurised milk
• Very few people in developed countries now acquire TB in this way, although meat and milk can still be a source of infection in some developing countries

Transmission via a Vector

Vectors are involved in the transmission of pathogens

A vector is any organism that transfers a pathogen from an infected individual to an uninfected individual

The vector themselves usually aren't harmed by the pathogen

Alot of disease vectors tend to be insects

Insects are ideal vectors as they reproduce in large numbers which increases the likelihood of pathogen transmission

Transmission of Malaria

Malaria is caused by one of four species of the protoctist Plasmodium

These protoctists are transmitted to humans by an insect vector:

Female Anopheles mosquitoes feed on human blood to obtain the protein they need to develop their eggs

If the person they bite is infected with Plasmodium, the mosquito will take up some of the pathogen with the blood meal

When feeding on the next human, Plasmodium pass from the mosquito to the new human's blood

Malaria may also be transmitted during blood transfusion and when unsterile needles are re-used

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