Topic 2 C: Cells and The Immune System Flashcards
What do proteins do on the cell surface membrane?
act as antigens.
What do antigens do?
allow the immune system to tell the difference between your own, healthy
body cells (known as ‘self’ cells) and ‘foreign’ invaders
What are antigens?
molecules (usually proteins) that can generate an immune
response when detected by the body
Where are antigens found?
usually found on the surface of cells, including all your body cells
What are foreign antigens?
Antigens that aren’t normally found in the body
What antigens does the immune system respond to?
foreign antigens
Antigens allow the immune system to identify:
Pathogens, Abnormal body cells, Toxins, Cells from other individuals of the same species
What are pathogens?
organisms that cause disease, e.g. bacteria, viruses and fungi.
All pathogens have antigens on their surface — these are identified as foreign
by immune system cells, which then respond to destroy the pathogen.
What are Abnormal body cells?
Cancerous or pathogen-infected cells have abnormal antigens on their surface, which trigger an immune response.
What are Toxins?
- poisons
- they are molecules, not cells.
Some toxins are produced by bacteria, e.g. the bacterium Clostridium botulinum releases a protein toxin that affects the nervous system, causing the symptoms of botulism. The immune system can respond to toxins, as well as the pathogens
that release them.
What happens when you receive cells from another person, such as in an organ transplant
or blood transfusion?
cells will have some antigens that are different
to your own (unless the donor is genetically identical to you)
- foreign antigens trigger an immune response. This response leads to the rejection of transplanted organs if drugs aren’t taken to suppress the recipient’s immune system.
What are the most important antigens for blood transfusions?
ABO blood group antigens
What happens if the donated blood contains A or B antigens that aren’t recognised by the recipient’s immune system?
Antigens will generate an immune response.
What are the main stages of the immune response?
- Phagocytosis
- T-Cells
- B-Cells
- Antibody production
What is a phagocyte?
E.g macrophage
- is a type of white blood cell that carries out
phagocytosis (engulfment of pathogens)
Where are phagocytes?
found in the blood and in tissues and are the first cells to respond to an immune system trigger inside the body.
What does a phagocyte do?
recognises the foreign antigens on a pathogen
What does the cytoplasm of the phagocyte do.
Moves round the pathogen, engulfing it
- so pathogen is now contained in a phagocytic vacuole in cytoplasm of the phagocyte.
What does a lysosome do with the phagocytic vacuole?
- is an organelle that contains enzymes called lysozymes
- fuses with the phagocytic vacuole.
The lysozymes break down the pathogen
What happens after the lysosome fuses with rhe phagocytic cell.
phagocyte presents the pathogen’s antigens — it sticks the antigens on its surface to activate
other immune system cells.
- The phagocyte is acting as an antigen‑presenting cell.
What are T cells?
- also called a T-lymphocyte
- type of white blood cell
- has receptor proteins on its surface that bind to complementary antigens
presented to it by phagocytes. This activates the T-cell.
What do different types of T-cells do?
- respond in different ways.
- e.g helper T-cells (TH cells) release chemical signals that activate and stimulate phagocytes and cytotoxic T-cells (TC cells), which kill abnormal and foreign cells. TH cells also activate B-cells, which secrete antibodies
What are B cells
- also called B-lymphocytes
- type of white blood cell
- They’re covered with antibodies — proteins that bind to antigens to form an antigen-antibody complex.
What does each B cell have?
different shaped antibody on its membrane, so different ones bind to different shaped antigens
Process of antibodies:
- antibody A will bind to antigen A as they have complementary shapes
-Antibody A will not bind to antigen B as they do not have complementary shapes
What is colonial selection?
Antibody meets a complementary shaped antigen + binds to it
- uses substances released from helper T-cells, activates the B-cell.
- The activated B-cell divides into plasma cells.
What are plasma identical to?
- B-cell (they’re clones)
What do plasma cells secrete?
loads of antibodies specific to the antigen.
(monoclonal antibodies)
What do monoclonal antibodies bind to?
antigens on the surface of the pathogen to form lots of antigen-antibody complexes
Why does an antibody have two binding sites?
can bind to two pathogens at the same time.
- so pathogens become clumped together (agglutination)
What happens after agglutination?
- phagocytes bind to antibodies + phagocytose many pathogens at once.
- leading the destruction of pathogens
carrying this antigen in the body
Description of structure of antibodies:
- Antibodies are proteins
- made up of chains of amino acids
- specificity of an antibody depends on its
variable regions, which form the antigen binding sites
What does each antibody have?
- a variable region with a unique tertiary structure (due to different amino acid sequences) that’s complementary to one specific antigen
- All antibodies have the same constant regions.
What are the two immune responses?
- Cellular
- Humoral
What forms the cellular response?
T-cells and other immune system cells that they interact with, e.g. phagocytes
What forms the Humoral response?
B-cells, clonal selection and the production of monoclonal antibodies form the humoral response.
Why are both immune responses needed?
remove a pathogen from the body and
the responses interact with each other
e.g. T-cells help to activate B-cells, and antibodies coat pathogens making it easier for phagocytes to engulf them.
What is the primary immune response?
When an antigen enters the body for the first time it activates the immune system.
Why is the primary response slow?
there aren’t many B-cells that can make the antibody needed to bind to it.
- Eventually the body will produce enough of the right antibody to overcome the infection.
What happens to a person during the primary response?
infected person will show symptoms of the disease.
When do memory cells form?
After being exposed to an antigen
- produced by T and B cells
How long do memory cells remain in the body?
a long time.
How does a person become immune to a disease?
Memory T-cells remember the specific antigen and will recognise it a second time round.
Memory B-cells record the specific antibodies needed to bind the antigen.
- so person is now immune
What does immune men?
A person’s immune system has the ability to
respond quickly to a second infection.
What is The secondary response?
If the same pathogen enters the body again, the immune system will
produce a quicker, stronger immune response
What happens faster in the secondary response?
- ## Clonal selection happens faster.
How does Clonal selection happen faster?
Memory T-cells are activated and divide into the correct type of T-cells to kill the cell carrying
the antigen.
What does the secondary response get rid of?
gets rid of the pathogen before you begin to show any symptoms
forms of immunity:
Active or passive
What is active immunity?
- a type of immunity you get when your immune system makes its own antibodies after being stimulated by an antigen.
two different types of active immunity:
- Natural - when you become immune after catching a disease.
- Artificial - when you become immune after you’ve been given a vaccination containing a harmless dose of antigen
What is Passive immunity?
- type of immunity you get from being given antibodies made by a different organism — your immune system doesn’t produce any antibodies of
its own.
Two different types of passive immunity:
Natural — this is when a baby becomes immune due to the antibodies it receives from its mother, through the placenta and in breast milk.
Artificial — this is when you become immune after being injected with antibodies from someone else. E.g. If you contract tetanus you can be injected with antibodies against the tetanus toxin, collected
from blood donations.
Difference between active and passive immunity: (active immunity)
Active immunity:
- Requires exposure to antigen
- takes a while for protection to develop
- Protection is long-term because the antibody is produced (after activation of memory cells) in response to complementary antigen being present in the body
Difference between active and passive immunity:
(Passive immunity)
- Doesn’t require exposure to antigen
- Protection is immediate
- Memory cells aren’t produced
- Protection is short-term as the antibodies given are broken down
What happens as B cells divide?
- while B cells divide to build up their numbers to deal with a pathogen (primary response), you suffer from the disease.
How do vaccinations help avoid disease?
- contain antigens that cause your body to produce memory cells against a particular pathogen, without the pathogen causing disease.
- so you become immune without getting any symptoms.
What is herd immunity?
- when vaccine reduces occurrence of the disease, those not vaccinated are also less likely to catch
the disease
What do vaccines contain?
- antigens (may be free or attached to a dead or attenuated (weakened) pathogen.
How can vaccines be put into the body?
- injected or taken orally.
disadvantages of taking a vaccine orally:
- could be broken down by enzymes in the gut or the molecules of the vaccine may be too large to be absorbed into the blood.
- Sometimes booster vaccines are given later on (e.g. after several years) to make sure that more memory cells are produced.
What are vaccines tested on before being tested on humans ?
- All vaccines are tested on animals
- some people disagree with animal testing
- animal based substances may be used to produce a vaccine, which some people disagree with
How is Testing vaccines on humans dangerous?
- volunteers may put themselves at unnecessary risk of contracting the disease as they think they’re fully protected
e.g. they might have unprotected sex as they
have had a new HIV vaccine and think they’re protected + vaccine might not work
Why do some people not want to take the vaccine?
- risk of side effects, but they are still protected because of herd immunity — other people
think this is unfair.
Why do people think not taking a vaccine is unfair?
If there was an epidemic of a new disease (e.g. a new influenza virus) there would be a rush to receive a vaccine and difficult decisions would have
to be made about who would be the first to receive it.
What activates the primary response?
Antigens on the surface of pathogens
- When you’re infected a second time with the same pathogen (which has the same antigens on its surface) they activate the secondary response and you don’t get ill
What is antigenic variation?
pathogens can change their surface antigens.
- Different antigens are formed due to changes in the genes of a pathogen.
What does antigenic variation lead to?
- when you’re infected for a second time, the memory cells produced from the first infection will
not recognise the different antigens. - immune system has to start from scratch and carry out a primary response against these new antigens.
- This primary response takes time to get rid of the infection, which is why you get ill again.
What does Antigenic variation make it difficult to do?
makes it difficult to develop vaccines against
some pathogens due to slow primary response
Examples of pathogens that show antigenic variation:
HIV and the influenza virus.
Why do influenza vaccines change?
- The influenza (flu) vaccine changes every year as antigens on the surface of the influenza virus change regularly, forming new strains of the virus.
How does antigenic variation affects the production of vaccines to help prevent people catching influenza?
- Memory cells produced from vaccination with one strain of the flu will not recognise other strains with different antigens. The strains
are immunologically distinct. Every year there are different strains of the influenza virus circulating in the population, so a different vaccine has to be made.
New vaccines are developed and one is chosen every year that is the most effective against the recently circulating influenza viruses.
Governments and health authorities then implement a programme of vaccination using the most suitable vaccine.
Difference in structure of antigens in 1st and second infection with antigenic variation:
1st infection - triggers a primary response
2nd infection - triggers a primary response with different antigens
What are Monoclonal antibodies?
- antibodies produced from a single group of
genetically identical B-cells (plasma cells) - meaning they’re all identical in structure.
What can monoclonal antibodies bind to?
- antibodies are very specific because their binding
sites have a unique tertiary structure that only an antigen with a complementary shape can fit into. - but monoclonal antibodies can bind to anything you want, e.g. a cell antigen or other substance, and they will only bind to (target) this molecule.
Why are monoclonal antibodies useful when binding to anything?
- can treat illnesses + used in medical diagnosis.
What do Different cells in the body have?
different surface antigens
What antigens do Cancer cells have?
tumour markers that are not found on normal body cells.
What can Monoclonal antibodies do to tumour makers?
- Monoclonal antibodies can be made that will bind to tumour markers.
- You can also attach anti-cancer drugs to the antibodies.
What happens when antibodies come into contact with the cancer cells?
bind to the tumour markers so drug will only accumulate in the body where there are cancer cells.
- So, the side effects of an antibody-based
drug are lower than other drugs as they accumulate near specific cells.
what do antibodies on Pregnancy tests do?
detect the hormone human chorionic gonadotropin (hCG) that’s found in the urine of pregnant women
- The application area contains antibodies that are complementary to the
hCG protein, bound to a coloured bead (blue).
When urine is applied to the application area any hCG will bind to
the antibody on the beads, forming an antigen-antibody complex.
The urine moves up the stick to the test strip, carrying any beads with it.
The test strip contains antibodies to hCG that are stuck in place
(immobilised).
If there is hCG present the test strip turns blue as the immobilised antibody binds to any hCG — concentrating the hCG-antibody complex with the blue beads attached. If no hCG is present, the beads will pass through the test area without binding to anything, and so it won’t go blue.
What can test strips with antibodies be used for?
medical diagnosis, e.g. malaria can be diagnosed using a test strip that detects malaria antigens in the blood.
What does ELISA stand for?
enzyme-linked immunosorbent assay
What does the ELISA test do?
allows you to see if a patient has any antibodies to a certain antigen or any antigen to a certain antibody.
What can the ELISA test be used for?
used in medical diagnosis to test for pathogenic infections (e.g. for of the immune system to HIV infection), for allergies (e.g. to nuts or lactose)
- and anything you can make an antibody for
What is used in an ELIZA test?
- an antibody is used which has an enzyme attached to it
- enzyme can react with substrate to produce coloured product, causing solution in reaction vessel to change colour
What does a colour change on the ELIZA test show?
- demonstrates that the antigen or antibody
of interest is present in the sample being tested (e.g. blood plasma) - In some types of ELISA, the quantity of this antigen/antibody can be worked out from the intensity of the colour change.
What is the simplest type of ELISA?
direct ELISA
Direct ELISA:
Antigens from a patient sample are bound to the inside of a well in a well plate
- A detection antibody (with an attached enzyme) that is complementary to the antigen of interest is added. If the antigen of interest is present in the patient sample, it will be immobilised on the inside surface of the well and the detection antibody will bind to it
What happens after the antibody binds in direct ELISA?
- binded antibody is washed out to remove any unbound antibody and a substrate solution is added.
If the detection antibody is present, the enzyme reacts with the substrate to give a colour change. This is a positive result for presence of the antigen
Why is Indirect ELISA is different from direct?
uses two different antibodies
indirect ELISA test for HIV:
- HIV antigen is bound to the bottom of a well in a well plate.
- sample of blood plasma (which might contain
several different antibodies, is added)
- if any HIV specific antibodies in plasma, they will bind to HIV antigen stuck to bottom of well. Well is washed out to remove unbound antibodies
What happens when well is washed out in indirect ELISA?
3.secondary antibody specific to enzyme attached is added
- this secondary antibody can bind to HIV specific antibody (primary) + well is washed out to remove unbound secondary antibody
- if there’s no primary antibody in sample, all secondary antibody will be washed away as there is nothing for it to bind to
What happens after a secondary antibody binds in well?
A solution is added to the well which contains a substrate which is able to react with the enzyme attached to the secondary antibody + produce coloured product
- if solution changes colour: patient has HIV specific antibodies in blood + is infected with HIV
Ethical issues surrounding the use of monoclonal antibodies:
- Animals are used to produce the cells from which the monoclonal antibodies are produced. Some people disagree with the use of animals in this way.
Why is it important that other scientists come up with more evidence for a new theory?
- validates theory
- Other scientists may repeat the study and try to reproduce the results, or conduct other studies to try to prove the same theory.
What is HIV?
human immunodeficiency virus
- a virus that affects the human immune system. It eventually leads to (AIDS).
What is AIDS?
- acquired immune deficiency syndrome
a condition where the immune system deteriorates and eventually fails. This makes someone with AIDS more vulnerable to other infections, like pneumonia.
What does HIV do to helper T-cells?
Acts as host cells
- Infects + kills helper T-cells
Why are Helper T-cells important in immune response?
send chemical signals that activate phagocytes, cytotoxic T-cells and B-cells
Without enough helper T-cells:
immune system is unable to mount an effective response to infections because other immune system cells don’t behave how they should.
When do People infected with HIV develop AIDS ?
when helper T-cell numbers in their body reach a critically low level.
During the initial infection period:
HIV replicates rapidly and the infected
person may experience severe flu-like symptoms. After this period, HIV replication drops to a lower level. (Latency period)
During the latency period (which can last for years) the infected person won’t experience any symptoms.
When are People with HIV are classed as having AIDS?
when symptoms of their failing immune system start to appear or their helper T-cell count drops below a certain level.
What is The length of time between infection with HIV and the development of AIDS?
varies between individuals but without treatment it’s usually around 10 years.
What type of diseases do people with AIDS generally develop?
diseases that wouldn’t cause serious problems in people with a healthy immune system.
initial symptoms of AIDS:
minor infections of mucous membranes (e.g. the inside of the nose, ears and genitals), and recurring
respiratory infections.
What happens as AIDS progresses?
number of immune system cells decreases further
- Patients become susceptible to more serious infections including chronic diarrhoea, severe bacterial infections and tuberculosis.
Patients During the late stages of AIDS:
- have a very low number of immune system cells and can develop range of infections e.g toxoplasmosis of the brain (a parasite infection) and candidiasis of the respiratory system (fungal infection).
It’s these serious infections that kill AIDS patients, not HIV itself.
Factors that affect progression of HIV to AIDS and survival time with AIDS:
existing infections, the strain of HIV they’re infected with, age and access to healthcare.
HIV structure:
- virus particle has a spherical structure.
- core contains genetic material (RNA) + some proteins e,g enzyme reverse transcriptase, which is needed for virus replication
Outer coating of HIV:
Has an outer coating of protein called a capsid and an extra outer layer called an envelope, which is made of membrane stolen from the cell membrane of a previous host cell.
Sticking out from the envelope are loads of copies of an attachment protein that help HIV attach to the host helper T-cell.
Where can HIV (and all other viruses) reproduce?
can only reproduce inside the cells of the organism
it has infected
- HIV replicates inside the helper T-cells of the host + doesn’t have the equipment (such as enzymes and ribosomes) to replicate on its own, so it uses those of the host cell.
HIV replicating:
- The attachment protein attaches to a receptor molecule on the cell membrane of the host helper T-cell.
- The capsid is released into the cell, where it uncoats and releases the genetic material (RNA) into the cell’s cytoplasm.
- Inside the cell, reverse transcriptase is used to make a complementary strand of DNA from the viral RNA template.
What happens after a strand of DNA for HIV replication is made?
- double-stranded DNA is made and inserted into the human DNA.
- Host cell enzymes are used to make viral proteins from the viral DNA found within the human DNA.
- The viral proteins are assembled into new viruses, which bud from the cell and go on to infect other cells.
What do antibiotics do?
kill bacteria by interfering with their metabolic reactions.
- target the bacterial enzymes and ribosomes used in these reactions.
What do antibiotics target?
- designed to only target the bacterial ones so they
don’t damage human cells. As bacterial enzymes and ribosomes are different from human enzymes
Ribosomes in viruses:
- Viruses don’t have their own enzymes and ribosomes
- they use ones in the host’s cells
Why can’t antibiotics inhibit bacteria in humans?
human viruses use human enzymes and ribosomes to replicate, antibiotics can’t inhibit them because they don’t target human processes.
What are antiviral drugs?
designed to target the few virus-specific enzymes (enzymes that only the virus uses) that exist
Example of HIV replicating:
HIV uses reverse transcriptase to replicate (see previous page). Human cells don’t use this enzyme, so drugs can be designed to inhibit it without
affecting the host cell.
These drugs are called reverse‑transcriptase inhibitors
What can slow down the progression of HIV infection and AIDS in an infected person?
- cure or vaccine for HIV
- antiviral drugs can be used to slow down the progression
The best way to control HIV infection:
- reducing its spread.
How can HIV be spread?
- unprotected sexual intercourse, through
infected bodily fluids (e.g. blood from sharing contaminated needles) and from a HIV-positive mother to her fetus. - Not all babies from HIV‑positive mothers are born infected with HIV and taking antiviral drugs during
pregnancy can reduce the chance of the baby being HIV-positive.
