A-LevelBiologyImmunity9700/21 · Oct/Nov 2019 · Q512 Marks

Myasthenia gravis is a condition that results in muscle weakness by affecting the immune response.

(a)Explain what is meant by the term immune response.[2]
(b)Myasthenia gravis is an autoimmune disease which disrupts a cell signalling pathway involving muscle cells.
(b)(i)Suggest how the immune system acts to disrupt this cell signalling pathway.[3]
(b)(ii)Enzyme Y is found in the cell surface membrane of muscle cells. Enzyme Y acts to break down the cell signalling molecules which trigger muscle contraction when they are no longer needed. Using the induced fit hypothesis of enzyme action, explain how enzyme Y breaks down the cell signalling molecules.[4]
(c)Rituximab is a monoclonal antibody used in the treatment of myasthenia gravis. Rituximab acts against the cell surface membrane protein CD20. This protein is found on the surface of the B-lymphocytes that cause myasthenia gravis. Explain the advantages of using monoclonal antibodies in the treatment of diseases such as myasthenia gravis.[3]

Mark scheme

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

[2] (a) two from ref.to non-self/ foreign (antigen); one from activation / AW, of lymphocytes ; ref. to specificity ; ref. to immunological memory ;
[3] (b)(i) three from (B-) lymphocytes respond / immune response, to self antigens (on muscle cell surface) / antibody produced by, B-lymphocytes / plasma cells; A failure to distinguish between self and non-self antibody binds to receptor; antibody has complementary shape to receptor; A idea of specificity cell signalling molecule / neurotransmitter / AW, unable to bind to receptor ; reactions / AW, within cells, not triggered / described ;
[4] (b)(ii) three from active site of enzyme Y is, not (fully) complementary / partially complementary ; active site, moulds to fit / becomes fully complementary to, cell signalling molecule; enzyme-substrate complex forms ; A ESC forms ref. to interaction between substrate and R groups of enzyme amino acids ; lowers activation energy; products leave the active site ; enzyme molecule unchanged / able to be re-used ; AVP; e.g. detail of how activation energy is lowered
[3] (c) three from I statements about locating or diagnosis of disease specific / targeted, therapy / treatment ; AW A only affects, one cell type in the body / B lymphocytes bind to / recognition of, receptors / antigens / CD20, on cell surface (of diseased cells) ; A in context of B lymphocytes or diseased cells monoclonal antibodies, are not recognised as foreign / do not trigger an immune response ; A ref. to humanisation of antibodies example of presence of monoclonal antibody stimulating the immune system / AW ; in context of examples given A stimulates phagocytosis / activates B-cells attach, radioactive substance / drug (to treat / kill, B lymphocytes / diseased cells); must be in context