immune system- 2C Flashcards
What is an antigen?
A molecule (usually a protein) on the surface of a cell that triggers an immune response.
What types of cells carry antigens?
Pathogens, abnormal body cells (e.g. cancerous), toxins, and cells from other individuals (e.g. organ transplants).
What is phagocytosis?
The process by which a phagocyte engulfs and digests a pathogen.
Outline the steps of phagocytosis.
- Phagocyte recognises antigens on pathogen
- Engulfs pathogen into a phagosome
- Lysosome fuses with phagosome, releasing lysozymes
- Pathogen is digested
- Antigen fragments are presented on the phagocyte’s surface.
What type of cell is the phagocyte after antigen presentation?
An antigen-presenting cell (APC).
What is a T-cell (T lymphocyte)?
A white blood cell with receptors that bind to specific antigens presented by APCs.
What are the main types of T-cells?
- Helper T-cells (TH): release cytokines to activate B-cells and phagocytes
- Cytotoxic T-cells (TC): kill infected cells by releasing perforin.
What is a B-cell (B lymphocyte)?
A white blood cell with antibodies on its surface that bind to specific antigens.
How are B-cells activated?
By binding to an antigen and receiving signals from helper T-cells (cytokines).
What happens after B-cell activation?
It divides into plasma cells and memory B-cells.
What do plasma cells do?
Produce lots of identical antibodies (monoclonal antibodies) specific to the antigen.
What is the structure of an antibody?
- Two variable regions (bind to antigens)
- Constant region (binds to immune cells)
- Disulfide bonds hold the structure together.
How do antibodies help destroy pathogens?
- Agglutination: clump pathogens together
- Bind to antigens, tagging pathogens for phagocytosis.
What is the primary immune response?
The first response to a new antigen — slower, symptoms usually occur.
What is the secondary immune response?
Faster and stronger due to memory cells — often no symptoms.
How do vaccines work?
They contain antigens (weakened/inactivated form) that trigger a primary immune response and memory cell production.
What is herd immunity?
When enough of the population is vaccinated, the spread of the disease is reduced, protecting unvaccinated individuals.
Why might vaccination not eliminate a disease?
- Antigenic variation
- Individuals may not respond
- Some may not be vaccinated.
What is antigenic variation?
When a pathogen mutates, changing the structure of its antigens.
Why is antigenic variation a problem?
Memory cells from previous infections/vaccines no longer recognise the pathogen — leads to reinfection.
Give an example of a pathogen with high antigenic variation.
Influenza virus (flu).
What are monoclonal antibodies?
Identical antibodies produced from a single B-cell clone, specific to one antigen.
Uses of monoclonal antibodies in medicine?
- Targeted cancer therapy
- Medical diagnosis (e.g. pregnancy testing, detecting hormones or infections).
What does ELISA stand for?
Enzyme-Linked Immunosorbent Assay.
What is the ELISA test used for?
To detect the presence of a specific antigen or antibody in a sample.
How does the ELISA test work (basic principle)?
- Antibody or antigen is bound to a plate
- Sample is added — if target is present, it binds
- A second antibody with an enzyme is added
- Substrate is added — colour change indicates a positive result.
What is HIV?
Human Immunodeficiency Virus — attacks and destroys helper T-cells.
What is AIDS?
Acquired Immune Deficiency Syndrome — a condition where the immune system is severely weakened due to HIV.
How is HIV transmitted?
Through infected bodily fluids — e.g. blood, semen, vaginal secretions, breast milk.
Why are antibiotics ineffective against viruses like HIV?
Viruses don’t have their own metabolism — they use host cells, so antibiotics (which target bacterial processes) have no effect.
