Influenza is an infectious disease caused by the influenza A virus. This virus causes influenza in birds and mammals. Fig. 5.1 is a diagram of an influenza A virus. Haemagglutinin allows the virus to attach to host cells by binding to receptors on the cell surface membrane of the host cells. Neuraminidase is an enzyme that helps the virus to leave host cells after the virus has replicated.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

1. 5(a) two from protein coat/capsid / capsomeres ; nucleic acid / DNA or RNA (core); I single- or double-stranded A 'DNA/RNA' I genetic material size given in nanometres / smaller than prokaryotes ; I small / very small / microscopic I acellular
2. 5(b) three from 1 drugs can be inhibitors (of neuraminidase); either 2 (competitive inhibitor) so binds to active site or (non-competitive inhibitor) so binds to allosteric site / AW or (drugs may act by) breaking down / hydrolysing / denaturing, all / part of neuraminidase ; A changes active site 3 no / less, enzyme-substrate complexes formed ; A ESC A substrate not able to bind to enzyme A neuraminidase cannot bind to (host cell) receptor 4 receptor is (still) complementary to haemagglutinin; 5 haemagglutinin becomes attached to, cell receptor / host cell (so newly formed virus does not leave the cells) ;
3. 5(c) four from 1 antigen presentation; in correct context 2 clonal selection / activation, of specific, B-lymphocytes / T-lymphocytes ; A B cells/T cells 3 (lymphocytes) divide by mitosis / undergo clonal expansion ; 4 B-lymphocytes, differentiate into / mature into / form / AW, plasma cells ; 5 antibodies secreted by plasma cells; 6 T-helper cells secrete cytokines; 7 cytokines stimulate / AW, B-lymphocytes / plasma cells / humoral response ;
4. 5(d)(i) max 2 (out of total three marks) advantages allow references to other pathogen types 1 antibodies are provided to people immediately / no delay for plasma cells to secrete antibodies ; A immediate, immunity / protection R immediate (immune) response 2 antibodies, immediately neutralise toxins / prevent viruses entering cells ; 3 prevents disease (in the individual) / promotes quicker recovery ; 4 prevents spread of the pathogen through the population / prevents people dying ; 5 antibodies can be manufactured quickly in response to mutations that occur in virus / AW ; max 2 (out of total three marks) disadvantages 6 short-term / temporary (immunity) ; 7 no memory cells produced ; 8 can have infections of same pathogen again ; 9 allergic reaction / immune response, to the (non-human) antibodies given ; 10 ref. to cost qualified ; e.g. needs to be repeated / high cost of production of antibodies 11 AVP - for advantage (A) or disadvantage (D) ; e.g. (A) passive can be used for people who are malnourished / immunosuppressed e.g. (A) ref. to using a vaccine with a (live) pathogen that might give person the disease
5. 5(d)(ii) across the placenta ; A via umbilical cord in breast milk / colostrum / breast feeding / during lactation ;