3.2.4 Cell recognition and the immune system Flashcards
Define antigen
- Molecules which, when recognised as non-self/foreign by the immune system, can stimulate an
immune response and lead to the production of antibodies - Often proteins/glycoproteins on the surface of cells
Why are antigens so specific what do they allow the immune system to identify ?
- Pathogens (disease causing organisms) e.g. viruses, fungi, bacteria
- Cells from other organisms of the same species e.g. organ transplant, blood transfusion
- Abnormal body cells e.g. cancerous cells / tumours
- Toxins released from bacteria
Describe the process of phagocytosis (Non-specific immune response )
- Phagocyte e.g. macrophage recognises foreign antigens on the pathogen and binds to
the antigen - Phagocyte engulfs pathogen by surrounding it with its cell surface membrane / cytoplasm
- Pathogen contained in vacuole/vesicle called a phagosome in cytoplasm of phagocyte
- Lysosome fuses with phagosome (phagolysosome) and releases lysozymes (hydrolytic
enzymes) into the phagosome - These hydrolyse / digest the pathogen
- Phagocyte becomes antigen presenting (APC) and stimulates specific immune response
Describe the cellular response (T-lymphocytes to a foreign antigen e.g. infected cells )
- T lymphocytes recognises antigen presenting cells after phagocytosis (foreign antigen)
- Specific T helper cell with receptor complementary to specific antigen binds to it, becoming
activated and dividing rapidly by mitosis to form clones which:
a) Stimulate B cells for the humoral response
b) Stimulate cytotoxic T cells to kill infected cells by producing perforin
c) Stimulate phagocytes to engulf pathogens by phagocytosis
Describe the humoral response (the response of B lymphocytes to a foreign antigen
e.g. in blood/tissues)
- Clonal selection:
a) Specific B cell binds to antigen presenting cell and is stimulated by helper T cells which
releases cytokines
b) Divides rapidly by mitosis to form clones (clonal expansion) - Some become B plasma cells for the primary immune response – secrete large amounts of
monoclonal antibody into blood - Some become B memory cells for the secondary immune response
Describe the primary response – antigen enters body
for the first time (role of plasma cells)
- Produces antibodies slower and at a lower
concentration because - Not many B cells available that can make
the required antibody - T helpers need to activate B plasma cells
to make the antibodies (takes time) - So infected individual will express symptoms
Describe the secondary response ( same antigen enters body again(role of memory cells )
- Produces antibodies faster and at a higher
concentration because - B and T memory cells present
- B memory cells undergo mitosis quicker / quicker
clonal selection
Define antibodies and describe how antibodies work
- Quaternary structured protein (immunoglobin)
- Secreted by B lymphocytes e.g. plasma cells and produced
in response to a specific antigen - Binds specifically to antigens (monoclonal) forming an
antigen-antibody complex - two binding sites so one antibody can bind to two
pathogens at a time (at variable region/binding site)
forming two separate antigen-antibody complex - Enables antibodies to clump the pathogens together –
agglutination = easier for engulfing
*the hinge regions aid binding to antigens
What is a vaccination ?
- Injection of antigens
- From attenuated (dead or weakened) pathogens
- Stimulates the formation of memory cells
- A vaccine can lead to symptoms because some of the pathogens might be alive / active /viable;
therefore, the pathogen could reproduce and release toxins, which can kill cells
How do vaccines provide herd immunity ?
- Large proportion but not 100% of population vaccinated against a disease – herd immunity
- Makes it more difficult for the pathogen to spread through the population because…
- More people are immune so fewer people in the population carry the pathogen / are infected
- Fewer susceptible so less likely that a susceptible / non-vaccinated individual will come into
contact with an infected person and pass on the disease
What are the ethical issues associated with the use of vaccines ?
- Tested on animals before use on humans → animals have a central nervous system so feel
pain (some animal based substances are also used to produce vaccines) - Tested on humans → volunteers may put themselves at unnecessary risk of contracting the
disease because they think they’re fully protected e.g. HIV vaccine so have unprotected sex
→ vaccine might not work - Can have side effects
- Expensive – less money spent on research and treatments of other diseases
What is antigen variability an explanation for ?
- New vaccines against a disease need to be developed more frequently e.g. influenza
- Vaccines against a disease may be hard to develop or can’t be developed in the first place eg. HIV
- May experience a disease more than once e.g. common cold
Explain the effect of antigen variability on disease
- Change in antigen shape (due to a genetic mutation)
- Not recognised by B memory cell → no plasma cells / antibodies
- Not immune
- Must re-undergo primary immune response → slower / releases lower concentration of antibodies
- Disease symptoms felt
Explain the effect of antigen variability on disease prevention (vaccines)
- Change in antigen shape (due to a genetic mutation)
- Existing antibodies with a specific shape unable to bind to changed antigens / form
antigen-antibody complex - Immune system i.e. memory cells won’t recognise different antigens (strain)
Evaluate methodology, evidence and data relating to the use of vaccinations
- A successful vaccination programme:
- Produce suitable vaccine
- Effective – make memory cells
- No major side effects → side effects discourage individuals from being vaccinated
- Low cost / economically viable
- Easily produced / transported / stored / administered
- Provides herd immunity
- Evaluating a conclusion that’s been made from a set of data / study
- If there is a scatter graph, the relationship between two variables may be a positive /negative
correlation, or no correlation - But correlation between two variables doesn’t always mean there’s a causal
relationship – correlation could be due to change or another variable / factor - Repeatability (when an experiment is repeated using the same method and equipment and
obtains the same results) - Have there been other experiments / studies showing the same?
- Validity (suitability of the investigative procedure to answer the question being asked)
- Does the data answer the question set out to investigate?
- Example: research project on potential vaccines to protect people against HIV used
monkeys and a virus called SIV (which only infects monkeys and causes a condition
similar to AIDS in them) . Scientists have questioned the value of the research because
there may be differences between human and money responses / immune systems,
and a vaccine developed against SIV may not work against HIV / may be (significant)
differences between SIV and HIV - Potential bias?
What are monoclonal antibodies ?
- Monoclonal antibody = antibody produced from a single group of genetically identical (clones)
B cells / plasma cells - Identical structure
- Bind to specific complimentary antigen
- Have a binding site / variable region with a specific tertiary structure / shape
- Only one complementary antigen will fit
Why are monoclonal antibodies useful in medicine ?
- Only bind to specific target molecules / antigens because…
- Antibodies have a specific tertiary structure (binding site / variable region) that’s complementary
to a specific antigen which can bind/fit to the antibody
Monoclonal antibodies: targeting medication to specific cell types by attaching
a therapeutic drug to an antibody
Example: cancer cell
1. Monoclonal antibodies made to be complementary to antigens specific to cancer cells → cancer
cells are abnormal body cells with different antigens (tumour markers)
2. Anti-cancer drug attached to antibody
3. Antibody binds / attaches to cancer cells (forming antigen-antibody complex)
4. Delivers attached anti-cancer drug directly to specific cancer cells so drug accumulates → fewer
side effects e.g. fewer normal body cells killed
Monoclonal antibodies: medical diagnosis
Example: pregnancy test
- Pregnant women have the hormone hCG in their urine
- Urine test strip has 3 parts with 3 different antibodies
- Application area, position 1: antibodies complementary to hCG (bound to a blue
coloured bead)
- Middle, position 2: antibodies complementary to hCG-antibody complex
- End, position 3: antibodies complementary to antibody without hCG attached
- If pregnant
- hCG binds to antibodies in application area = hCG-antibody complex
- Travels up test strip, binds to antibodies at position 2 = blue line
- If not pregnant
- No hCG in urine so hCG doesn’t bind to antibodies in application area so doesn’t
bind to antibodies at position 2 = no blue line
- Bind to antibodies at position 3 → blue line = control
What is the ELISA test used for ?
To determine if a patient has
a) Antibodies to a certain antigen
b) Antigen to a certain antibody
What are ethical issues associated with the use of monoclonal antibody ?
- Animals are involved in the production of monoclonal antibodies i.e. by producing cancer
in mice who have a CNS so feel pain, and it is unfair to give them a disease - Although effective treatment for cancer and diabetes has caused deaths when used in
treatment of Multiple Sclerosis - Patients need to be informed of risk and benefits before treatment so they can make
informed decisions
How does HIV work/replicated ?
- HIV infects T helper cells (host cell)
- HIV attachment protein (GP120) attaches to a receptor on
the helper T-cell membrane - Virus lipid envelope fuses with cell surface membrane and capsid
released into cell which uncoats, releasing RNA and reverse
transcriptase into cytoplasm - Viral DNA is made from viral RNA
- Reverse transcriptase produces a complementary viral DNA
strand from viral RNA template
- Double stranded DNA is made from this (DNA polymerase) - Viral DNA integrated into host cell’s DNA (by enzyme integrase)
- This remains latent for a long time in host cell until activated
- Host cell enzymes used to make viral proteins from viral DNA
(within human DNA) → viral proteins assembled with viral RNA to
make a new virus - New virus bud from cell (taking some of cell surface membrane as
envelope) - Eventually kills helper T cells
- Most host cells are infected and process repeat
How does HIV cause AIDS ?
- Infects and kills helper T cells (host cell) as it multiplies rapidly
- T helper cells then can’t stimulate cytotoxic T cells, B cells and phagocytes →
impaired immune response - E.g. B plasma cells can’t secrete antibodies for agglutination and destruction
of pathogens by phagocytosis - Immune system deteriorates
- More susceptible to infections
- Diseases that wouldn’t cause serious problems in healthy immune system are deadly
(opportunistic infections) e.g. pneumonia
Why are antibiotics ineffective against viruses ?
- Antibiotics can’t enter human calls – but viruses exists in its host cell (they are acellular)
- Viruses don’t have own metabolic reactions e.g. ribosomes (use of the host cell’s) which
antibiotics target - If we did use them… act as a selection pressure + gene mutation = resistant strain of
bacteria via natural selection → reducing effectiveness of antibiotics and waste money
