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Biology Paper 1 > Topic 2: Infection and Response > Flashcards

Topic 2: Infection and Response Flashcards

1
Q

What is an infection?

A

The interaction between a pathogen and the body’s various defence mechanisms.

Sometimes the pathogen overwhelms the defences and the individual dies, and vice versa

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2
Q

What is a pathogen?

A

A disease-causing microorganism

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3
Q

What is immunity?

A

The ability of organisms to resist infection by protecting against pathogens or their toxins

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4
Q

Describe the two types of defence mechanism. Include examples

A
  • Non specific: response is immediate and the same for all pathogens, can be a physical barrier (e.g skin) or phagocytes
  • Specific: response is slower and specific to each pathogen, can be the cell-mediated response (T lymphocytes) or the humoral response (B lymphocytes)
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5
Q

Name 4 examples of barrier defence mechanisms of the human body

A
  • Skin
  • Mucus
  • Hydrochloric acid
  • Lysozymes
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6
Q

Describe how skin defends against pathogens

A

The outer layer contains flat, dead cells mostly made from the protein keratin,
Forms a tough barrier that microorganisms can’t penetrate

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7
Q

Describe how mucus defends against pathogens

A

The nose and gas exchange system are lined with cilia (hair-like structures) bathed in mucus.
Pathogens that are breathed in get stuck in the mucus. Cilia waft it up to the throat, where it is swallowed and killed by HCl in the stomach

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8
Q

Describe how hydrochloric acid defends against pathogens

A

The stomach produces HCl with a pH of 1.5-3.5. It kills the pathogens in food and drink, and from mucus swallowed from the gas exchange system. The vagina isn’t as acidic (pH 4-4.5) but it still inhibits the entry of pathogens

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9
Q

Describe how lysozymes protect against pathogens

A

They’re enzymes that digest the cell walls of bacteria. Found in tears, saliva and the mucus of the respiratory tract

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10
Q

What is a lymphocyte?

A

White blood cells involved in the immune response

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11
Q

What is a self antigen?

What is a foreign antigen?

A

A molecule of the cell surface membrane of your body cells that doesn’t trigger an immune response

A molecule on the cell surface membrane of a ‘foreign’ cell that does trigger an immune response

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12
Q

What is it that lets the immune system recognise cells?

A

Every type of cell has molecules in its cell-surface membrane, but the proteins are the most important because they have enormous variety and a highly specific tertiary structure

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13
Q

What kind of substances get recognised by lymphocytes?

A
  • Pathogens
  • Abnormal body cells (e.g cancer cells)
  • Toxins (including ones produced by pathogens)
  • Non-self material (e.g cells from other organisms of the same species)
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14
Q

Why is the immune system problematic for tissue/organ transplants?
How have we adapted to this?

A

The immune system recognises them as foreign and tries to destroy them.
Means transplants are normally matched as closely as possible to the recipient, e.g relatives that are closely genetic
Immunosuppressant drugs reduce the immune response

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15
Q

Are lymphocytes first produced after an infection?

A

No - there are 10 million types of lymphocytes already in the body, so the lymphocyte specific to the pathogen will already exist.
The complementary lymphocyte builds up its numbers until it can be effective (clonal expansion)

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16
Q

How do lymphocytes learn to recognise the body’s own cells?

A
  • In the fetus, the 10 million types of lymphocytes constantly collide with other cells
  • Infection in the fetus is rare because it is protected by the placenta, so lymphocytes only collide with ‘self’ material
  • Some lymphocytes have complementary receptors to body cells - these die or are suppressed
  • Only remaining lymphocytes are those that fit foreign material
  • Lymphocytes produced in adult bone marrow initially only encounter self antigens. Those that show an immune response undergo programmed cell death (apoptosis) before they differentiate and enter the blood
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17
Q

Describe what happens in the first immune response after pathogens gain entry to the body

A

Phagocytes (the type of white blood cell other than lymphocytes) carry out phagocytosis, where they engulf and destroy the pathogen before it can cause harm. They are present in the blood but can migrate to other tissues

Damaged tissue releases a chemical called histamine, causing blood vessels to dilate and speed up the arrival of phagocytes to the site.

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18
Q

Describe the steps of phagocytosis

A
  • Chemical products (cytokines) of the pathogen act as attractants, so the phagocyte moves towards it
  • The pathogen attaches to a receptor on the cell-surface membrane of the phagocyte
  • The phagocyte engulfs the pathogen to form a vesicle called a phagosome - its plasma membrane is from the cell surface membrane of the phagocyte
  • Lysosomes fuse to the phagosome, releasing hydrolytic enzymes that hydrolyse the molecules making up the pathogen into smaller soluble products (it is digested)
  • Soluble products are absorbed into the phagocytes cytoplasm and the pathogen’s antigens are presented on the cell surface membrane
  • Inflammation occurs at the site of infection due to the release of histamine. The swollen area contains dead pathogens + phagocytes (pus)
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19
Q

What is an antigen?

A

Any part of an organism / substance recognised as non-self (foreign) and stimulates an immune response. Usually part of the cell-surface membrane or cell wall

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20
Q

What is the cell mediated immune response?

A

T lymphocytes only respond to antigens presented on a body cell (rather than antigens in body fluids). The receptors on a T cell respond to 1 type of antigen, but there are many types.

It is specific, slower than non specific responses but can provide long-term immunity. Depends on lymphocytes produced by stem cells in bone marrow

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21
Q

Describe the two types of lymphocytes

A
  • T lymphocytes - mature in the thymus gland, involved in cell mediated immunity, don’t secrete antibodies
  • B lymphocytes- mature in bone marrow, involved in humoral immunity, secrete antibodies
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22
Q

What are antigen presenting cells?

A

Cells that display foreign antigens on their surface so T lymphocytes can distinguish between invader cells and normal cells

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23
Q

Describe 4 types of antigen presenting cell

A
  • Phagocytes that have engulfed and hydrolysed a pathogen presents its antigens on the cell surface membrane
  • Body cells invaded by a virus present some viral antigens on their cell surface membrane
  • Transplanted cells from individuals of the same species have different antigens
  • Cancer cells are different from normal body cells so present antigens on their cell surface membrane
24
Q

Describe the steps of cell mediated immunity

A
  • Pathogens invade body cells and are engulfed by phagocytes
  • The phagocyte presents the pathogen’s antigens on its cell surface membrane
  • Receptors on a specific helper T cell are specific to the antigen
  • The attachment activates the T cell to divide rapidly by mitosis and form clones
  • The cloned T cells complete a variety of tasks
25
Q

Describe the 4 things that cloned T cells could do after cell mediated immunity

A
  • Develop into memory cells that enable a rapid response to future infections by the same pathogen
  • Stimulate phagocytes to engulf pathogens by phagocytosis
  • Stimulate B cells to divide and secrete their antibody
  • Activate cytotoxic T cells to kill abnormal cells and infected body cells by producing a protein called perforin that makes holes in the cell surface membrane. It becomes freely permeable so the cell dies.
    Most effective against viruses because they replicate in living cells
26
Q

What is humoral immunity?

A

Immunity that involves antibodies which are soluble in the blood and tissue fluid of the body.

Each pathogen has a variety of antigens on its surface, each is complementary to a different protein receptor on a B lymphocyte so a variety of B lymphocytes are involved in response to 1 pathogen. Each toxin molecule it produces also acts as an antigen.

27
Q

Describe the steps of humoral immunity

A
  • Pathogen antigens circulating bloodstream are taken up by a B lymphocyte by endocytosis
  • B cell processes antigens + presents them on its surface
  • Activated helper T cells attach to presented antigens + activate B cell (all of above is clonal selection)
  • B cell is activated to divide by mitosis to make plasma cell clones (clonal expansion)
  • Plasma B cells produce + secrete the complementary antibody
  • Antibodies released into blood + bind with antigens on pathogen forming antigen-antibody complexes (lead to pathogen’s destruction)
  • Some B cells develop into memory cells which can respond to future infection by same pathogen by dividing rapidly + developing into plasma B cells
28
Q

Describe the primary immune response

A

Slower than secondary response + produces less antibody

Plasma cells only survive for a few days but produce 2000 antibodies per second. Produces memory cells

29
Q

Describe the secondary immune response

A

Rapid and of greater intensity than the primary response.

Memory cells can live for decades circulating in body fluid. They don’t produce antibodies directly but when they find the pathogen, divide rapidly and develop into plasma and memory cells. Provides long-term immunity

30
Q

What is an antibody?

A

A protein with a specific binding site, synthesised by B cells.
They react with an antigen on the surface of foreign material by binding to them.
They are proteins due to the huge variety in tertiary structure.

31
Q

Describe the structure of an antibody

A
  • Y-shaped
  • Have 2 identical antigen binding sites, which are complementary to the antigen and can form antigen-antibody complexes
  • Made from 4 polypeptide chains - the long ones are heavy chains, the short are light
  • Polypeptide chains held together by disulphide bridges
  • The 2 variable regions at the binding sites differ between antibodies
  • Have a constant region which is the same for each antibody, this binds to protein receptors
32
Q

How do antibodies lead to the destruction of an antigen?

A

Antibodies don’t destroy antigens directly, but just prepare them for destruction by:
- Causing agglutination - antibodies have 2 binding sites so each can form 2 antigen-antibody complexes. Many pathogens are joined, forming clumps of antigens easier for phagocytes to find (as they are less spread-out in the body)
- Serve as markers that stimulate phagocytes to engulf the pathogen to which they’re attached

33
Q

What are polyclonal antibodies?

A

Many different antibodies are cloned (many different types of antigen are present on a pathogen, so there are many types of B cell effective against it, so many antibodies are produced)

34
Q

What are monoclonal antibodies?

A

When you isolate and clone a single type of antibody to form a single type of B plasma cell clone

35
Q

Give three uses of monoclonal antibodies

A
  • Targeted medication, e.g cancer treatment (antibodies are particular to a specific antigen so monoclonal antibodies can target specific cell types)
  • Pregnancy testing
  • Medical diagnosis, e.g ELISA test
36
Q

Describe direct monoclonal antibody therapy

A
  • MaBs are produced that are specific to antigens on cancer cells
  • Antibodies are given to the patient and attach to receptors on the cancer cells
  • They attach to the surface of cancer cells and block chemical signals that stimulate their uncontrolled growth
  • The MaBs are not toxic and highly specific, causing fewer side effects
37
Q

Describe indirect monoclonal antibody therapy

A
  • Cancer cells have antigens on them that aren’t on normal body cells, called tumour markers
  • MaBs that bind to tumour markers are attached to a radioactive or cytotoxic drug (a drug that kills cells)
  • MaBs bind to the tumour markers and kill the cells
  • They are specific so the drug can have smaller doses, which is cheaper and causes less side effects
38
Q

Describe how pregnancy testing works

A
  • The placenta produces the hCG hormone, which is found in the mother’s urine
  • Urine passes through reaction zone, hCG hormone binds to mobile hCG antibody forming antigen-antibody complexes. The antibodies are linked to coloured particles
  • Sample passes up stick where the hCG binds to immobilised hCG antibodies in results zone
  • Other antibodies not attached to hCG bind to antibodies in the control zone
  • Dye appears in both control + results zones to show a positive result
39
Q

What is the ELISA test?

A

Enzyme Linked Immunosorbant Assay

Uses antibodies to detect the presence and quantity of a protein in a sample. A very sensitive test

40
Q

Describe the steps of the direct ELISA test

A
  • Antigens from patient sample are bound to the inside of a well
  • A detection antibody complementary to the target antigen is added. It has an enzyme attached to it
  • Well is washed to remove any unbound antibodies
  • A solution is added - contains a substance that reacts to the attached enzyme to produce a coloured product (positive result)
41
Q

Describe the steps of the indirect ELISA test

A

Often used for HIV
- HIV antigen bound to bottom of a well in a well plate
- Sample of patient’s blood plasma added. Any HIV- specific antibodies in plasma will bind to HIV at the bottom of the well.
- Well is washed to remove any unbound antibodies
- Secondary antibody attached to an enzyme is added to the well + binds to the HIV-specific antibody
- Well is washed to remove unbound secondary antibodies
- Colourless substrate of the enzyme is added - their reaction makes a coloured product
- Amount of antigen present is relative to the intensity of the colour

42
Q

Discuss the ethics of monoclonal antibodies

A
  • Producing them involves deliberately inducing cancer in mice
  • Have been used to successfully treat diseases (e.g diabetes, cancer) but have caused deaths from the treatment of multiple sclerosis - important that patient has full knowledge of the risks (informed consent)
  • Testing for the safety of new drugs can be dangerous for the volunteers
43
Q

Describe how monoclonal antibodies are produced

A
  • Mouse injected with target antigen - its B lymphocytes produce antibodies
  • Myelomas (malignant white blood cell tumour cells in bone marrow) can divide quickly but not produce antibodies
  • B cells removed from mouse’s spleen + are mixed with myelomas
  • Detergent added to mixture to break down cell-surface membranes + fuse them into hybridomas
  • Hybridoma cells can divide + make antibodies
  • Hybridomas cloned in a culture medium all producing same antibody. Any unfused myeloma cells can’t survive in medium + die
  • Large amounts of monoclonal antibodies are collected + purified by centrifugation, filtration + chromatography
44
Q

Describe passive immunity

A

The introduction of antibodies from an outside source. No direct contact with the pathogen/antigen is needed. Immunity is acquired immediately.

Antibodies not replaced by individual + memory cells not produced. Gives short-term protection because antibodies will break down.

Natural passive immunity = maternal antibodies
Artificial passive immunity = monoclonal antibodies

45
Q

Describe active immunity

A

Produced by stimulating the production of antibodies by the individual’s own immune system. Direct contact with the pathogen/antigen is necessary.

Immunity takes time to develop + is usually long-lasting (antibodies are replaced by individuals + memory cells are produced)

Natural active immunity = infection under normal circumstances
Artificial active immunity = vaccination (immunisation)

46
Q

What is a vaccine?

A

A preparation / injection of antigens or attenuated (whole) microorganisms that have been treated in some way that makes it harmless.
Stimulates the formation of memory cells so any new infection will be rapidly overcome before it can cause harm.
On a large scale, it can cause herd immunity

47
Q

Give some features of a successful vaccination program

A
  • Suitable vaccine must be economically available in sufficient quantities to immunise the most vulnerable population
  • Must be few side effects, if any (discourages people from getting it)
  • Means of producing, storing and transporting it must be available
  • Must have the means to administer the vaccine properly at the appropriate time
  • Must be possible to vaccinate the vast majority of the vulnerable population to produce herd immunity
48
Q

What is herd immunity?

A

When a sufficiently large proportion of the population has been vaccinated to make it difficult for a pathogen to spread

When most people are immune, it is unlikely that a susceptible individual will come in contact with an infected person, so it also protects unvaccinated people. Important because it is impossible to vaccinate everyone (e.g can’t vaccinate babies and the immunocompromised)

49
Q

Why might vaccination not eliminate a disease?

A
  • Vaccination fails to induce immunity in certain individuals
  • Individuals could develop disease immediately after vaccination, before their immunity levels are high so they can still spread it
  • Antigenic variability - pathogen could mutate + change antigens, making the vaccine ineffective
  • May be so many varieties of a pathogen that it is impossible to develop a vaccine against them all
  • Certain pathogens may ‘hide’ from immune system e.g inside cells + in places out of reach (intestine)
  • Individuals may object the vaccine for religious, ethical + medical reasons
50
Q

Discuss the ethics of vaccines

A
  • How acceptable is animal testing during production?
  • How to balance side effects against the risk of disease?
  • On whom should they be tested?
  • Is it ok to make the vaccine compulsory for the benefit of the population?
  • Should expensive vaccination programmes continue when the disease is almost eradicated?
  • How can individual health risks from vaccines be balanced against controlling the disease?
51
Q

What is HIV?
How is it transmitted and how do these relate to preventions?

A

Human Immunodeficiency Virus

  • Unprotected intercourse (condoms)
  • Blood transfusions (blood screening)
  • Sharing needles between intravenous drug users (supplying clean needles)
52
Q

Describe the structure of HIV

A

Has the structure of a typical virus
Belongs to the group of retroviruses - they have two strands of RNA and the reverse transcriptase enzyme to make DNA from RNA

53
Q

Describe the replication of HIV

A
  • HIV enters bloodstream + circulates the body
  • Attachment protein on HIV binds to a protein called CD4 - most commonly on helper T cells
  • Protein capsid fuses with cell-surface membrane . RNA + enzymes enter the cell
  • HIV reverse transcriptase converts virus’ RNA into DNA
  • DNA enters the T helper cell’s nucleus via the nuclear pore + is inserted into its DNA
  • HIV DNA creates messenger RNA with cell’s enzymes (instructions for making new viral proteins)
  • mRNA passes out nucleus via nuclear pore + uses cell’s protein synthesis mechanisms to make HIV particles
  • HIV particles break away from the helper T cell with a piece of its cell-surface membrane as its lipid envelope
54
Q

What is AIDS?
How does it come about?
What does it cause?

A

Acquired Immune Deficiency System

HIV often stays dormant + causes AIDS many years later. HIV virus attacking helper T cells decreases cell-mediated immunity, and so humoral immunity as well - weakens immune system

Body is unable to produce an adequate immune response, so becomes susceptible to infections and cancers.
HIV doesn’t kill people, neither does AIDS, only a secondary infection that the body can’t fight off

55
Q

Why do antibiotics not work on viruses?

A

Many antibiotics work by targeting the formation of murein cell walls in bacteria - viruses have a protein coat instead so it has no effect.

When viruses are inside host cells, antibiotics can’t reach them.

Viruses rely on host cells to carry out metabolic activities, so they lack metabolic pathways + cell structures for antibiotics to target.

Biology Paper 1 (9 decks)

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