cell recognition +immune system Flashcards
cells topic
What is an antigen?
● Foreign molecule / protein / glycoprotein / glycolipid
● That stimulates an immune response leading to production of antibody
How are cells identified by the immune system?
● Each type of cell has specific molecules on its surface (cell-surface membrane / cell wall) that identify it
● Often proteins → have a specific tertiary structure (or glycoproteins / glycolipids)
What types of cells and molecules can the immune system identify?
- Pathogens (disease causing microorganisms) eg. viruses, fungi, bacteria
- Cells from other organisms of the same species (eg. organ transplants)
- Abnormal body cells eg. tumour cells or virus-infected cells
- Toxins (poisons) released by some bacteria
- injected antigens e.g. vaccines
- our own cells if autoimmune
Describe phagocytosis of pathogens (non-specific immune response)
- Phagocyte attracted by chemicals / recognises (foreign) antigens on pathogen
2.Phagocyte engulfs pathogen by surrounding it with its cell membrane - Pathogen contained in vesicle / phagosome in cytoplasm of phagocyte
- Lysosome fuses with phagosome and releases lysozymes (hydrolytic enzymes)
- Lysozymes hydrolyse / digest pathogen
what does phagocytosis lead to the presentation of?
Phagocytosis leads to presentation of antigens where antigens are displayed on the phagocyte cell-surface
membrane, stimulating the specific immune response (cellular and humoral response)
Describe the response of T lymphocytes to a foreign antigen (the cellular response)
-T lymphocytes recognise (antigens on surface of) antigen presenting cells eg. infected cells, phagocytes
presenting antigens, transplanted cells, tumour cells etc.
- made in the thymus gland
-Specific helper T cells with complementary receptors (on cell surface) bind to antigen on
antigen-presenting cell → activated and divide by mitosis to form clones which stimulate:
● Cytotoxic T cells → kill infected cells / tumour cells (by producing perforin)
● Specific B cells (humoral response - see below)
● Phagocytes → engulf pathogens by phagocytosis
- become memory T-cells which do the above upon reinfection
describe the specific response
-leads to immunity- long term resistance
- there are 2 types:
1. cellular(in cell) response (call mediated immunity) for use on foreign cells
2. humoral response (anything that travels in the blood) for use on non-cellular foreign bodies
what are the possible targets of cells?
- foreign transplanted cells
- a virus infected cell
- cancer cells
- phagocytes that are presenting foreign antigens
diagram of the cellular response
what is the humoral response?
- carried out by the B-lymphocytes (made in the bone marrow)
- can destroy pathogens outside the cells
- leads to antibody production
- they need some help from helper T-cells
Describe the response of B lymphocytes to a foreign antigen (the humoral response)
-B lymphocytes can recognise free antigens eg. in blood or tissues, not just antigen presenting cells.
1. Specific B lymphocyte with complementary receptor (antibody on cell surface) binds to antigen via endocytosis
2. B-lymphocyte presents foreign antigen on cell membrane
3.Th will bind to presented antigen and trigger mitosis
4.This is then stimulated by helper T cells (which releases cytokines)
5. So divides (rapidly) by mitosis to form clones
6. Some differentiate into B plasma cells → secrete large amounts of (monoclonal) antibody
7. Some differentiate into B memory cells → remain in blood for secondary immune response
diagram of B- cell response
What are antibodies?
● Quaternary structure proteins (4 polypeptide chains)
● Secreted by B lymphocytes eg. plasma cells in response to specific antigens
● Bind specifically to antigens forming antigen-antibody complexes
describe the structure of antibodies
Explain how antibodies lead to the destruction of pathogens
● Antibodies bind to antigens on pathogens forming an antigen-antibody complex
○ Specific tertiary structure so binding site / variable region binds to complementary antigen
● Each antibody binds to 2 pathogens at a time causing agglutination (clumping) of pathogens
● Antibodies attract phagocytes
● Phagocytes bind to the antibodies and phagocytose many pathogens at once
Explain the differences between the primary and secondary immune response
● Primary - first exposure to antigen
○ Antibodies produced slowly & at a lower conc.
○ Takes time for specific B plasma cells to be
stimulated to produce specific antibodies
○ Memory cells produced
● Secondary - second exposure to antigen
○ Antibodies produced faster & at a higher conc.
○ B memory cells rapidly undergo mitosis to
produce many plasma cells which produce
specific antibodies
Describe the differences between active and passive immunity
Describe the structure of a HIV particle
Explain the use of antibodies in the ELISA (enzyme-linked immunosorbent
assay) test to detect antigens
- testing strip contains HIV antigens
- blood adding to sample
- if HIV antibodies are present, they bind to test strip
- first wash - removes blood
- add secondary antibody which contains enzyme that is complemenatry to first antibody
- second wash to remove second antibody and enzyme
- add dye that changes colour in presence of enzymes
- colour change : blue to green
Explain the use of antibodies in the ELISA test to detect antibodies
Example method (indirect ELISA):
1. Attach specific antigens to well
2. Add sample with potential antibodies, wash well
3. Add complementary monoclonal antibodies
with enzymes attached → bind to antibodies if
present
4. Wash well → remove unbound antibodies
5. Add substrate → enzymes create products that
cause a colour change (positive result)
