2.4 Cell Recognition and the Immune System

Question 1

what is an antigen

Answer 1

-a foreign molecule/protein
-that stimulates an immune response
-leading to the production of antibodies

Question 2

how are cells identified by the immune system

Answer 2

-each cell has specific molecules on its surface that identify it
-these molecules are often proteins with a specific tertiary structure

Question 3

what types of cells and molecules can the immune system identify

Answer 3

-pathogens
-cells from other organisms of the same species
-abnormal body cells
-toxins released by some bacteria

Question 4

what is a pathogen

Answer 4

-a disease causing microorganism

Question 5

describe phagocytosis of pathogens (non specific immune response)

Answer 5

-phagocyte is attracted to chemoattractants from pathogen/ recognises antigens on pathogen
-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
-lysozymes hydrolyse pathogen
-phagocytes absorb products of hydrolysis

Question 6

what does phagocytosis lead to

Answer 6

-presentation of antigens on the phagocyte cell surface membrane
-stimulating the specific immune response

Question 7

describe the response of t lymphocytes to a foreign antigen (the cellular response)

Answer 7

-t lymphocytes recognise antigen presenting cells
-specific helper T cells with complementary receptors bind to antigen on the antigen presenting cell
-the antigen presenting cell is activated and divides by mitosis to form clones

Question 8

what do the clones produced in the t lymphocyte response stimulate

Answer 8

-cytotoxic cells which kill infected cells
-specific b cells
-phagocytes

Question 9

describe the response of b lymphocytes to a foreign antigen (humoral response)

Answer 9

-specific b lymphocyte with complementary receptor binds to antigen
-this is stimulated by helper T cells which releases cytokines
-so divides rapidly by mitosis to form clones (clonal selection)
-differentiate into b plasma or b memory cells

Question 10

what can b lymphocytes recognise

Answer 10

-free antigens, not just antigen presenting cels

Question 11

what do b plasma cells do

Answer 11

-secrete large amounts of antibodies

Question 12

what do b memory cells do

Answer 12

-remain in the blood for the secondary immune response

Question 13

what are antibodies

Answer 13

-quaternary structure proteins
-secreted by b lymphocytes in response to specific antigens
-bind specifically to antigens forming antigen-antibody complexes

Question 14

describe the structure of an antibody

Answer 14

-variable region
-constant region
-heavy polypeptide chain
-light polypeptide chain
-antigen binding site
-hinge region
-disulfide bridge

Question 15

explain how antibodies lead to the destruction of pathogens

Answer 15

-antibodies bind to antigens on pathogens forming an antigen-antibody complex
-as they have a specific tertiary structure so the binding site is complementary to the antigen
-each antibody binds to 2 pathogens at a time causing agglutination of pathogens
-antibodies attract phagocytes
-phagocytes bind to the antibodies and phagocytose many pathogens at once

Question 16

describe the primary immune response

Answer 16

-first exposure to antigen
-antibodies produced slowly and at a lower concentration
-takes time for specific b plasma cells to be stimulated to produce specific antibodies
-memory cells are produced

Question 17

describe the secondary immune response

Answer 17

-second exposure to antigen
-antibodies produced faster and at a higher concentration
-b memory cells rapidly undergo mitosis to produce many plasma cells which produce specific antibodies

Question 18

what is a vaccine

Answer 18

-injection of antigens from attenuated pathogens
-stimulating formation of memory cells

Question 19

explain how vaccines provide protection to individuals against disease

Answer 19

-specific b lymphocyte with complementary receptor binds to antigen
-specific t helper cell binds to antigen presenting cell and stimulates b cell
-b lymphocyte divides by mitosis to form clones
-some differentiate into b plasma cells which release antibodies
-some differentiate into b memory cells
-on second exposure to antigen, b memory cells divide rapidly by mitosis to produce b plasma cells
-these release antibodies faster and at a higher concentration

Question 20

explain how vaccines provide protections for populations against disease

Answer 20

-herd immunity
as
-large proportion of population immune so don’t become ill from infection
-fewer infected people pass pathogen on/unvaccinated people less likely to come into contact with someone with the disease

Question 21

what is herd immunity

Answer 21

-large proportion of population vaccinated, reducing spread of pathogen

Question 22

describe active immunity

Answer 22

-initial exposure to antigen
-memory cells involved
-antibody produced and secreted by b plasma cells
-slow, takes longer to develop
-long term immunity as antibody can be produced in response to a specific antigen again

Question 23

describe passive immunity

Answer 23

-no exposure to antigen
-no memory cells involved
-antibody introduced from another organism eg breast milk/placenta
-faster acting
-short term immunity as antibody hydrolysed

Question 24

explain the effect of antigen variability on disease and disease prevention

Answer 24

-antigens on pathogens change shape/tertiary structure due to gene mutations (creating new strains)
-so no longer immune
-b memory cell receptors cannot bind to changed antigen on secondary exposure
-specific antibodies not complementary to changed antigen

Question 25

examples of antigen variability affecting diseases

Answer 25

-yearly flu vaccinations developed -no vaccine for HIV -can catch a cold many times

Question 26

describe the structure of a HIV particle

Answer 26

-lipid envelope -RNA -reverse transcriptase -capsid -attachment protein

Question 27

describe the replication of HIV in helper T cells

Answer 27

-HIV attachment proteins attach to the receptors on helper T cell -lipid envelope fuses with cell surface membrane, releasing capsid into cell -capsid uncoats, releasing RNA and reverse transcriptase -reverse transcriptase converts viral RNA to DNA -viral DNA inserted/ incorporated into helper T cell DNA (may remain latent) -viral protein/capsid/enzymes are produced a) DNA transcribed into HIV mRNA b) HIV mRNA translated into new HIV proteins -virus particles assembled and released from cell (via budding)

Question 28

explain how HIV causes the symptoms of AIDS

Answer 28

-HIV infects and kills helper T cells (host cell) as it multiplies rapidly -so T helper cells cant stimulate cytotoxic T cells, B cells and phagocytes -so B plasma cells can't release as many antibodies for agglutination and destruction of pathogens -immune system deteriorates so more susceptible to infections -pathogens reproduce, release toxins and damage cells

Question 29

explain why antibiotics are ineffective against viruses

Answer 29

viruses do not have structures/ processes that antibiotics inhibit: -viruses do not have metabolic processes eg do not make protein/ribosomes -viruses do not have bacterial enzymes/murein cell wall

Question 30

what is a monoclonal antibody

Answer 30

-antibody produced from genetically identical/cloned B lymphocytes/plasma cells -so have same tertiary structure

Question 31

explain how monoclonal antibodies can be used in medical treatments

Answer 31

-monoclonal antibody has a specific tertiary structure/variable region -complementary to receptor/protein/antigen found only on a specific cell type -therapeutic drug attached to antibody -antibody binds to specific cell, forming antigen-antibody complex, delivering drug

Question 32

what are some monoclonal antibodies designed to do other than being attached to a therapeutic drug

Answer 32

-block antigens/receptors on cells

Question 33

explain how monoclonal antibodies can be used in medical diagnosis

Answer 33

-monoclonal antibody has a specific tertiary structure/variable region -complementary to specific receptor/protein/antigen associated with diagnosis -dye/stain/fluorescent marker attached to antibody -antibody binds to receptor forming antigen-antibody complex

Question 34

explain the use of antibodies in the ELISA test to detect antigens (direct)

Answer 34

-attach sample with potential antigens to well -add complementary monoclonal antibodies with enzymes attached that will bind to antigens if present -wash well to remove unbound antibodies that could cause a fake positive -add substrate - enzymes create products that cause a colour change (positive result)

Question 35

explain the use of antibodies in the ELISA test to detect antigens (sandwich)

Answer 35

-attach specific monoclonal antibodies to well -add sample with potential antigens, then wash well -add complementary monoclonal antibodies with enzymes attaches which bind to antigens if present -wash well to remove unbound antibodies that could cause a false positive -add substrate - enzymes create products that cause a colour change

Question 36

explain the use of antibodies in the ELISA test to detect antibodies (indirect)

Answer 36

-attach specific antigens to well -add sample with potential antibodies, wash well -add complementary monoclonal antibodies with enzyme attached which bind to antibodies if present -wash well to remove unbound antibodies -add substrate as enzymes create products that cause a colour change

Question 37

suggest the purpose of a control well in the ELISA test

Answer 37

-compare to test to show only enzyme causes colour change -compare to test to show all unbound antibodies have been washed away

Question 38

suggest why failure to thoroughly wash the well can result in a false positive on the ELISA test

Answer 38

-antibody with enzyme remains/not washed out -so substrate converted into colour product

Question 39

discuss some general ethical issues associates with the use of vaccines and monoclonal antibodies

Answer 39

-pre clinical testing on/ use of animals - potential harm/stress but animals not killed and helps produce drugs to reduce human suffering -clinical trials on humans - potential harm/side effects -vaccines - may continue high risk activities and still develop/pass on pathogen -use of drug- potentially dangerous side effects

Question 40

suggest some points to consider when evaluating methodology relating to the use of vaccines and monoclonal antibodies

Answer 40

-was the sample size large enough to be representative -were participants diverse in terms of age, sex, ethnicity and health -were placebo/control groups used for comparison -was the duration of the study long enough to show long term effects -was the trial double blind to reduce bias

Question 41

what is a double blind trial

Answer 41

-neither doctor/patient knew who was given drug or placebo

Question 42

suggest some points to consider when evaluating evidence and data relating to the use of vaccines and monoclonal antibodies

Answer 42

-what side effects observed and how frequently -was a statistical test used - difference in start and final results -was the standard deviation of final results large -did standard deviations of start and final results overlap -what dosage was optimum -does increasing dose increase effectiveness enough to justify extra cost -was the cost of production and distribution low enough