What is a pathogen and examples

Pathogen: Disease causing organism

Bacteria - E. Coli

  • Prokaryotes (no real nucleus)
  • Divide by Binary Fusion
  • Food poisioning (Salmonella)
  • Ear and eye infections
  • Cholera and diarrhea

Viruses - HIV/Influenza

  • Acelullar (non-living)
  • Needs a host (cell to carry out functions of life incl reproduction)
  • Can have DNA or RNA
  • Mutate, evolve and recombine quickly
  • Ebola, Herpes, Smallpox
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Pathogen examples continued

Fungi - Epidermophyton (Athlete's Foot)

  • Eukaryotes - reproduce with spores
  • Athlete's Foot, Mould, Ring Worm
  • Allergic reactions and respirtory problems

Protozoa - Malaria Parasite

  • Simple parasites
  • Malaria
  • Toxoplamosis
  • Leishmaniasis
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Methods of Transmission

Inhaled droplets - Influenza virus

Direct Contact - Herpes (virus), Varicella (virus)

Bodily Fluids - Strep throat, HIV

Animal Vectors - Rabies (virus), Malaria (Protozoa)

Blood Contact - Hepitatis B (virus)

Ingested/Swallowed - Salmonella (bacteria)

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Primary Defence


  • Continuous (hard to find an opening)
  • Many layers
  • Dry
  • pH is not favourable to pathogens
  • Lysozomes - enzymes breakdown pathogens
  • Natural organisms - competitive exclusion by non-harmful microbes

Mucous Membranes

  • Sticky mucus traps invaders
  • pH not favourable to pathogens
  • Lysozome - enzymes breakdown pathogens
  • Natural organisms - competitve exclusion by non-harmful microbes
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How can we protect ourselves

Prevent from coming into contact with them through qaurantine


  • Outer layers are tough providing a physical barrier 
  • Sebaceous glands secrete lactic acid and fatty acids making skin surface more acidic - this prevents most pathogenic growth

Mucous Membrane

  • Soft areas of skin kept moist with mucus
  • Found in nose, trachea, urethra, vagina
  • Bacteria are killed by Lysozomes (enzymes in mucus)
  • In trachea pathogens are caught in sticky mucus cillia then pushes bacteria and mucus up and out
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Blood clotting prevents blood loss and infection

Platelets: Cell fragments formed in bone marrow

Circulate in blood (along with red and white blood cells)

No nucleus

Lasts 8-10 days

If skin is cut 

1) Prothrombin and Fibrinogen (plasma proteins) and platelets circulate in the blood

2) When blood vessel broken blood escapes from the closed system

3) Damaged cells of blood vessel release chemicals causing platelets to adhere

4) Damaged tissues and platelets release clotting factors that convert prothrobin into thrombin

5) Thrombin convert soluble Fibrinogen into insoluble Fibrin

6) Fibrin is a fibrous protein that forms a mesh-like network arcross the wound to stablise the platelet plug

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Scar creation process continued

7) Blood cells are caught in the mesh and soon and eventually becomes a semi-solid clot

8) If exposed to air - the clot dries to form a protective scab and remains there unti the wound is healed

  • Wounds cause an opening to the skin through which pathogens can potentially enter the body
  • Platelets (small cell fragments) and damaged cell tissues release clotting factors in response to a wound
  • Clotting factors cause a series of reactions which end with Fibrin (protein) fibres forming a mesh across the wound site
  • The fibrin fibres catch blood cells and platelets - forming a clot
  • In the presence of air the clot dries to form a scab - seals the healing wound underneath
  • Blood clots at wounds prevent blood loss and the entry of pathogens (infections)
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Blood clots within Coronary Artery

If plaques in coronary artery rupture - Coronary Thrombosis: Artery completely blocked

Cardiac muscle gets no oxygen and stops beating in a rhythmic manner which can lead to a heart attack

 Fibrillation: Uncoordinated contraction of cardiac muscle

(Think of Difibrillator that used to schock heart back into normal rhythym)

Can recover but severe heart attacks are fatal

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Phagocytes & Non-Specific Immunity

Phagocytes ingest pathogens by endocytosis

Pathogens are then killed and digested by enzymes from Lysosomes

Antigens on the surfaces of cells and viruses allow for recognition

If a cell is not recognised - Phagocyte engulfs and ingests the pathogen by phagocytosis and lysosomes digests the pathogen

Non-specific Immunity: Does not distinguish between pathogens (ingests any pathogen)

Phagocytes are in the blood and squeeze out through the walls of capiliaries and move through tissues to infection sites

If Phagocyte response is not enough and infection spreads - Lymphocytes are called in to help

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Lymphocytes & Specific Immunity

Antigens: Molecule found on cell or virus surface that causes antibody formation

Antibodies: Globular protein that recognizes a specific antigen and binds to it as part of an immune response

Antibodies bind to Antigens

Antibodies are specific to Antigens

  • Antigens stimulate the production of Antibodies
  • Each Lymphocyte can only make one type of antibody. Each Lymphocyte puts some of the antibodies it can make on the cell surface membrane
  • When a pathogen enters its antigen binds to the antibody of that Lymphocyte
  • This makes the Lymphocyte active and divides by mitosis and produces Plasma cells: Clones of identical cells
  • Plasma cells produce a lot of the same antibody
  • Antibody binds to the antigens on the surface of the pathogens and stimulates its destruction
  • After the infection most Lymphocytes dissapear but some are kept as memory cells
  • Memory Cells: Quickly reproduce to form plasma cells if a pathogen with the same antigen is encountered again
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Production of Antibodies

  • Many different Lymphocytes exist 
  • Each type recognises one specific Antigen
  • When the immune system is challanged by a pathogen the corresponding Lymphocyte responds
  • It makes many clones of itself each of which produce Antibodies to the pathogen
  • This process is called clonal selection as the right lymphocyte is selected and then cloned
  • Some clone cells remain as Memory Cells ready for a second invasion by the pathogen
  • This is immunity
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What are Antibiotics

Antibiotics: Chemicals that selectively block processes that occur in prokaryotic cells (bacteria) but not in eukaryotic cells (human or animal cells)

Why do antibiotics have no effect on viruses?

  • Virus has no metabolism of its own - they rely on a host cell (human cell) to carry out metabollic processes
  • Not possible to block viral processes without also harming the host cell

Taking Antibotics for Colds and Flu? [Colds and Flus are viruses]

  • No point - Viruses use host cell metabolism
  • Viruses are protected by the host cell structure
  • Viruses have a different structure to prokayrotes (protein capsid and genetic material no cell wall or membrane to attack)
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Bacterial Resistance to Antibiotics

There is genetic variation amongst bacteria who can reproduce quickly. Most bacterial diseases can be treated with Antibiotics 


Some bacteria acquire genes that are resistant to antibiotics

Bacteria with a mutation for resistance has an advantage in the presence of antibiotics

These types of bacteria can reproduce quickly leading to a resistant strain of bacteria

  • High number of bacteria. Few of them are resistant to antibotics
  • Antibiotics kill bacteria causing the illness as well as good bacteria that protects the body from infection
  • The resistant bacteria now have preffered conditions to grow and take over
  • Bacteria can transfer their drug resistance to other bacteria causing more problems
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Florey and Chain test Penicillin on Mice

Penicillin was developed as an antibotic by Florey and Chain 

First Test: 8 mice infected with bacterium that causes fatal pneumonia

  • Four treated mice recovered
  • Four untreated mice died

Second Test: Man close to death from Bacterial Infection

  • Small quantities of impure penicillin was tested on a man close to death
  • He started to recover but the antibotic ran out

Thrid Test: Five patients testedAll patients were cured

Florey and Chain's experiments would not be regarded as safe today

  • Extensive animal testing is done to check for harmful effects
  • Smaller and larger doses are then tested on healthy and informed humans to see if drug is tolerated
  • Drug is tested on patients with disease - if small scale trials effective then larger double blind experiments are carried out to test drug effectiveness and side effects
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Modern Drug Testing and Clinical Trials

  • Years of extensive labratory research on animal and human cells to determine usefulness, likely dosage and side-effects
  • Asses the safety of the drug and side-effects impact on small group of healthy volunteers (small dosage which is increased)
  • Test effectiveness of drug against a control on a small group of affected volunteers against placebo and  competing drugs
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HIV and the Immune System

Human Immunodeficiency virus: Infects lymphocyte that plays a vital role in antibody production

Overtime lymphocytes are gradually destroyed when antibodies are not produced HIV develops into AIDS

If left untreated death can be caused by infections (pathogens would normally be easily controlled)


  • Skin contact                                                                                       HIGEST RISK
  • Toilet seats
  • Mosquitoes                                                                                        Sex and Blood Contact               


  • Kissing/saliva and oral sex
  • Ingestion
  • Child birth and breast feeding
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How is HIV Transmitted

Transmission through bodily fluids

  • Sexual Intercourse
  • Sharing needles with traces of blood
  • From mother to baby through the placenta or milk during breastfeeding
  • Transfused blood
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