Cell Recognition Flashcards
What is an antigen?
- Foreign molecule / protein/ glycoprotein / glycolipid
- That stimulates animmune responseleading to production ofantibody
How are cells identified by the immune system?
- Each type of cell hasspecific moleculeson itssurface(cell-surface membrane / cell wall) that identify it
- Oftenproteins→have aspecific tertiary structure(or glycoproteins / glycolipids)
Antigen Variability
- Some pathogens exhibit antigen variability:
- Antigens on their surface change frequently due to mutations
- Results in new strains of the pathogen
- Consequences of antigen variability:
- The specific immune response becomes ineffective
- Antibodies from lymphocytes and memory cells can no longer bind to the altered antigen
- Pathogen can evade recognition by the immune system
What types of cells and molecules can the immune system identify?
- Pathogens(disease causing microorganisms) eg. viruses, fungi, bacteria
- Cells from other organismsof the same species (eg. organ transplants)
- Abnormal body cellseg. tumor cells or virus-infected cells
- Toxins(poisons) released by some bacteria
Describe phagocytosis of pathogens (non-specific immune response)
1) Phagocyte attracted by chemicals/ recognise (foreign) antigens on pathogen.
2)Phagocyte engulfs pathogen by surrounding it with its cell membrane
3)Pathogen contained in vesicle/ phagosome in cytoplasm of phagocyte
4) Lysosome fuses with phagosome and released lysozymes
5) Lysozymes hydrolyse/ digest pathogens
After phagocytosis what occurs in the phagocyte
Phagocytosis leads topresentation of antigenswhere antigens are displayed on the phagocytecell-surface membrane, stimulating thespecificimmune response (cellular and humoral response).
Describe the response of T lymphocytes to a foreign antigen (the cellular response)
B lymphocytes can recognisefreeantigens eg. in blood or tissues, not just antigen presenting cells.
Specifichelper T cells with complementary receptors(on cell surface) bind to antigen onantigen-presenting cell→activated and divide by mitosis to form clones whichstimulate:
- Cytotoxic T cells→kill infected cells / tumour cells (by producing perforin)
- SpecificB cells(humoral response - see below)
- Phagocytes→engulf pathogens by phagocytosis
Describe the response of B lymphocytes to a foreign antigen (the humoral response)
Clonal selection:
- Specific B lymphocytewithcomplementary receptor(antibody on cell surface) binds toantigen
- This is then stimulated byhelper T cells(which releases cytokines)
- So divides (rapidly) bymitosisto formclones
- Some differentiate intoB plasma cells→secrete large amounts of (monoclonal)antibody
- Some differentiate intoB memory cells→remain in blood for secondary immune response
What are antibodies?
- Quaternary structure proteins(4 polypeptide chains)
- Secreted byB lymphocyteseg. plasma cells in response tospecificantigens
- Bindspecificallyto antigens formingantigen-antibody complexes
Explain how antibodies lead to the destruction of pathogens
Antibodiesbind to antigenson pathogens forming anantigen-antibody complex
○Specifictertiary structuresobinding site / variable region bindstocomplementary antigen
- Each antibody binds to2 pathogensat a time causingagglutination(clumping) of pathogens
- Antibodiesattract phagocytes
- Phagocytes bind to the antibodies andphagocytose many pathogens at once
Explain the differences between the primary & secondary immune response
- Primaryfirst exposure to antigen
- Antibodies producedslowly& at alower conc.
- Takes time for specificB plasma cellsto be stimulated to produce specific antibodies
- Memory cells produced
- Secondarysecond exposure to antigen
- Antibodies producedfaster& at ahigher conc.
- B memory cellsrapidly undergo mitosis to produce manyplasma cellswhich produce specific antibodies
What is a vaccine?
- Injection ofantigensfromattenuated(dead or weakened) pathogens
- Stimulating formation ofmemory cells
Explain how vaccines provide protection to individuals against disease
- SpecificB lymphocytewith complementary receptor binds toantigen
- SpecificT helper cellbinds toantigen-presentingcell andstimulatesB cell
- B lymphocyte divides bymitosisto form clones
- Some differentiate intoB plasma cellswhich releaseantibodies
- Some differentiate intoB memory cells
- Onsecondary exposureto antigen,B memory cellsrapidly divide by mitosis to produceB plasma cells
- These releaseantibodies fasterand at ahigher concentration
Explain how vaccines provide protections for populations against disease
Herd immunity-large proportionof population vaccinated, reducingspreadof pathogen
- Large proportion of populationimmunesodo not become illfrom infection
- Fewer infectedpeople to passpathogenon /unvaccinatedpeopleless likelyto come incontact with someone with disease
Explain the effect of antigen variability on disease and disease prevention
- Antigens on pathogenschange shape / tertiary structuredue togene mutations(creating new strains)
- Sono longer immune(from vaccine or prior infection)
- B memory cellreceptors cannot bind to / recognise changed antigen on secondary exposure
- Specific antibodiesnot complementary / cannot bindto changed antigen
