4.1 Difficult Model Answers

Question 1

Phagocytosis

Answer 1

• Engulf and digest pathogens
• Receptor on phagocyte’s cell surface membrane binds to antigen on pathogen’s cell surface membrane
• Pathogen engulfed by endocytosis
• Phagosome produced
• Lysosomes fuse w phagosome
• Lysins released
• Pathogen hydrolysed –> amino acids and fatty acids
• Products absorbed into cytoplasm by diffusion

Question 2

Structure + function of antibodies

Answer 2

• 4 polypeptide chains, disulphide bonds, Y shaped molecule
• Constant region - binding to phagocytes
• Variable region - comp. in shape to antigen - binds to antigens
• More than 1 variable region - allows attachment to more than one antigen and therefore more than one pathogen (agglutination)
• Hinge region - flexibility of branches - allows binding to more than one antigen

Question 3

Roles of antibodies

Answer 3

• Binding to antigens on pathogens
• Neutralising pathogens
• Antibodies cover binding sites on pathogens –> prevents entry to host cell
• Agglutination of pathogens - multiple variable regions allows agglutination of pathogens - clump too large to enter host cell and inc. likelihood of being consumed by phagocyte

Question 4

Immune response - T lymphocytes

Answer 4

• Macrophages engulf and digest pathogen’s antigens –> antigen presenting cell - helps w selecting correct T lymphocytes w receptors complimentary to antigens = clonal selection
• Once correct T lymphocytes selected - divide by mitosis = clonal expansion
• T helper - release interleukins w specific shape to bind to comp. receptors on cell surface membrane of B lymphocytes, stimulating them to divide by mitosis and differentiate, also stimulates macrophages to carry out more phagocytosis
• T killer - kill infected host cells, secrete protease enzymes into them
• T memory - stay in blood in case of second infection by same pathogen, allow faster secondary response- recognise antigen, can make clones and differentiate to form new T cells more quickly

Question 5

Changes and roles of B lymphocytes in an immune response (humoral response)

Answer 5

• Specific B lymphocytes w receptors comp. in shape to the antigens on the invading pathogen selected = clonal selection
• Cytokines released by T helper cells stimulate B lymphocytes to undergo mitotic division in clonal expansion and to differentiate into plasma cells and B memory cells
• Plasma cells - produce and secrete antibodies comp. in shape to the antigen, cause agglutination or neutralisation of the pathogens
• B memory cells - stay in blood in case of second infection w same pathogen, allow faster secondary response - recognise antigen, can make clones and differentiate to form plasma cells and so antibodies are made more quickly

Question 6

Examples of cell signalling in immune response

Answer 6

• Pathogen’s antigens communicate to body cells that they are foreign
• Infected cells displaying foreign antigens communicate to lymphocytes to be selected in clonal selection and to T killer cells that they need to be killed
• Antigen presenting cells communicate to T lymphocytes to be selected in clonal selection
• T helper cells release interleukins which bind to receptors on B lymphocytes and stimulate them to undergo mitotic division and differentiate

Question 7

Primary and secondary immune response

Answer 7

Primary:

• Time delay to trigger immune response
• No B memory cells - slow antibody production, few produced

Secondary:

• Shorter delay
• Much quicker response
• B memory cells specific to the antigen of the pathogen in blood, clone and differentiate to make plasma cells which make antibodies much more quickly - give immunity

Question 8

Why are the elderly etc encouraged to get an influenza vaccine yearly?

Answer 8

• Vaccine changed yearly
• Mutations –> new strains
• New strains = different antigens
• Old antibodies no longer complimentary to new antigen
• New vaccine encourages new antibodies to be made