cell recognition and the immune system Flashcards
Define antigens
Molecules that can generate an immune response when detected by the body
Define pathogens
Organisms that cause disease
What organisms can cause disease?
- Abnormal body cells (cancer cells, pathogen-infected cells)
- Toxins
- Cells from other individuals of the same species (organ transplants)
What are the 3 main stages of the immune response
- Phagocytes engulfing pathogens
- Phagocytes activating T-cells
- T-cells activating B-cells that divide into plasma cells
Define a phagocytes
A phagocyte is a type of white blood cell that carries out phagocytosis
Define phagocytosis
The engulfment of pathogens
Where are phagocytes found
In the blood and tissues
What are the first cells to respond to an immune system trigger inside the body?
Phagocytes
Describe how phagocytes work
- A phagocyte recognises the foreign antigens on a pathogen
- The cytoplasm of the phagocyte moves around the pathogen, engulfing it
- The pathogen is contained in a phagocytotic vacuole in the cytoplasm of the phagocytes
- A lysosome fuses with the phagocytotic vacuole
- The lysozymes break down the pathogens
- The phagocyte presents the pathogen’s antigens, it sticks the antigens on its surface to activate other immune system cells
What do phagocytes activate?
T-cells
Describe T-cells
A type of white blood cells that has receptor proteins on its surface that bind to complementary antigens presented to it by phagocytes
How does a T-cell become activated?
When it binds to the complementary antigens presented by phagocytes
What is the function of helper-T-cells?
Release chemical signals that activate and stimulate phagocytes and cytotoxic T-cells
What is the function of cytotoxic T-cells
They kill abnormal and foreign cells
What do T-cells activate?
B-cells
Describe B-cells
A type of white blood cell that is covered with antibodies which bind to antigens to form an antibody-antigen complex
How is each B-cell specific
Each B-cell has a differently shaped body on its membrane, so different ones bind to complementary shaped antigens
How do B-cells work?
- When the antibody on the surface of a B-cell meets a complementary-shaped antigen, it binds to it
- This activates the B-cell by clonal selection
- The activated B-cell divides into plasma cells
What are plasma cells?
They are clones of the B-cells
What is the function of plasma cells
They secrete loads of antibodies specific to the antigens such as monoclonal antibodies
Define antibodies
Antibodies are proteins that bind to pathogens
What is the function of an antibody?
Antibodies bind pathogens together, this allows phagocytes to engulf many pathogens at once
What determines the specificity of an antibody?
- It depends on its variable regions
- Each antibody has a variable region with a unique tertiary structure that’s complementary to one specific antigen
What is the function of a variable region on an antibody?
To form an antigen binding site
What determines the shape of the variable region of an antibody?
Different amino acid sequences
What is the same in all antibodies?
The constant region
Describe the structure of an antibody
- It is a protein made up of chains of amino acids
- Has variable regions which form antigen-binding sites which are complementary to one specific antigen
- They have the same constant region as each other
What does the formation of an antigen-antibody complex lead to?
Agglutination and phagocytosis
How many binding sites does an antibody have?
2 binding sites
Describe agglutination
- When two pathogens bind to the binding site of 1 antibody at the same time
- They become clumped together
What is the advantage of agglutination?
- It allows phagocytes to bind to the antibodies and phagocytose many pathogens at once
- This leads to the destruction of the pathogens carrying the antigen in the body
Define 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
Because there aren’t many B-cells that can make the antibody needed to bind to it
What is produced at the end of the primary response?
- After being exposed to an antigen
- T-cells and B-cells produce memory cells, these remain in the body for a long time
Describe memory T-cells
They remember the specific antigens and will recognise it second time round
Describe memory B-cells
Record the specific antibodies needed to bind to the antigen
Define the secondary immune response
If the same pathogen enters the body again, the immune system can produce a quicker and stronger response
Why is the secondary response fast
Because clonal selection occurs faster so the right antibody to the antigen is produced at a faster rate
Define clonal selection
The process of matching the antigens on antigen-presenting cells with the antigen receptors on B and T lymphocytes
Describe clonal selection
- Memory B-cells are activated and divide into plasma cells that produce the right antibody for the antigen
- Memory T-cells are activated and divided into the correct type of T-cells to kill the cell carrying the antigen
Describe and explain how a secondary immune response differs from a primary immune response
- A secondary immune is a faster and stronger response than the primary immune response
- This is because memory cells are produced during the primary response, which are able to recognise the foreign antigen when it is encountered again
- During the second infection, memory B-cells can quickly divide to form plasma cells, which secrete the correct antibody to the antigen
- Memory T-cells quickly divide into the right type of T-cell to kill the cell carrying the antigen
How do vaccines work?
- They contain antigens that cause your body to produce memory cells against a particular pathogen, without the pathogen causing disease
- So you become immune without symptoms
Describe herd immunity
If a large number of the population is vaccinated, it reduces the occurrence of the disease so that those who are not vaccinated are less likely to catch the disease
What do vaccines always contain?
Antigens that cause your body to produce memory cells against a particular pathogen
How might vaccines be taken?
- Injection
- Taken orally
What are the disadvantages of taking a vaccine orally?
- It could be broken down by enzymes in the gut
- The molecules of the vaccine may be too large to be absorbed into the blood
What is the function of a booster vaccine?
To make sure that memory cells are produced
Why do vaccines give long-term immunity?
Because they produce memory cells that can produce complimentary antibodies to the antigen
How are pathogens prepared for vaccines made harmless?
- Killing
- Weakening (e.g. heating)
- Purified antigens (removed from a pathogen)
- Using toxoids which are inactivated toxins that trigger an immune response
What are the ethical issues with vaccinations
- All are tested on animals
- Humans in clinical trials may put themselves at risk
- Some people refuse vaccines over fears of side effects but are produced due to herd immunity in the same way as people who cannot get the vaccine
- If there was a new disease, a difficult decision would be made about who would be the first to receive it
Why do vaccinations not fully eliminate disease?
- Fails to induce immunity in some people due to immunodeficiency
- People get infected before enough memory cells are generated by vaccine to afford protection - slow primary response
- Antigenic variability - pathogens can mutate
- Many different strains - we cant vaccinate against all
- Objections to vaccination based on moral, religious and ethical grounds
What activates the primary response
Antigens on the surface of a pathogen
Define antigen variability / antigenic variation
Different antigens are formed on the surface of a pathogen due to changes in the gene of the pathogen
What happens in the immune response when antigenic variability occurs
- When the pathogen infects again
- Memory cells produced from the primary response don’t recognise the different antigens
- So a new primary response has to occur to produce memory cells for the new antigens
Define active immunity
The immunity you get when your immune system makes its own antibodies after being stimulated by an antigen
What are the main differences between passive and active immunity?
- Active immunity requires exposure to the antigen, passive doesn’t
- Active immunity takes a while for protection to develop, passive immunity is immediate
- Active immunity produces memory cells, passive memory cells aren’t produced
- Active immunity protection is long-term because the antibody is produced after activation of memory cells in response to the complementary antigen being present in the body, passive immunity offers short-term protection because the antibodies given are broken down
Define monoclonal antibodies
They are antibodies produced from a single group of genetically identical B-cells
Describe how monoclonal antibodies are used in pregnancy tests
- Antibodies are complimentary to the HCG protein and are bound to a coloured bead
- HCG in urine binds to antibodies
- Urine moves up the strip carrying beads
- Immobilised antibodies which bind to HCG create the first blue line
- Immobilised antibodies that bind to coloured beads create a second blue line if HCG is present
What is the function of the ELISA test
Can be used to test for the presence of any antigen or antibody
Describe how the ELISA test works in general
- An antibody is bound to an enzyme, this enzyme reacts with a substrate to produce a colour change
- The antibody will be complementary and bind to the antigen or an antibody the test is designed to detect
- In the washing step, any unbound antibodies plus the attached enzyme are removed
- so there are more antigens in the sample therefore there will be more colour change because more enzymes will be present to react with the substrate
What is HIV?
A virus that affects the immune system and can lead to AIDS
How does HIV affect the immune system?
- The virus infects helper T cells to prevent an immune response
- It replicates in T cells using their cellular machinery, eventually, the cells swell and bursts
- Over time the number of T cells decreases and eventually the individual will develop AIDS
Describe the structure of HIV
- It has a core that contains genetic material as RNA and reverse transcriptase
- Has a capsid that is an outer coating of protein
- It has an envelope that is an extra outer layer made of a membrane stolen from the cell membrane of a previous host cell
- Has attachment protein on its surface which helps the HIV attach to the host helper T-cell
Describe how HIV replicates
- The attachment protein attaches to a receptor molecule on the cell membrane of the host T-helper cell
- The capsid is released into the cell, where it uncoats and releases RNA into the cytoplasm
- Reverse transcriptase is used to make a complementary strand of DNA from the viral RNA template
- The double-stranded DNA is made and inserted into human DNA
- Host cell enzymes are used to make viral proteins from the viral DNA found within the human DNA
- The viral protein are assembled into new viruses which bud from the cell and infect other cells
How do antibiotics kill bacteria?
Kill bacteria by interfering with their metabolic reactions, they target the bacterial enzymes and ribosomes used in these reactions
Explain why don’t antibiotics kill HIV?
- Viruses don’t have their own enzymes and ribosomes, they use the host’s cells
- Antibiotics can’t inhibit them because they don’t target human processes
What drugs treat viruses?
Antiviral drugs
How is HIV spread
- Sharing blood
- Unprotected sexual intercourse
- HIV mother to her fetus
