A-LevelBiologyImmunity9700/21 · May/June 2018 · Q510 Marks

The toxins released by some pathogenic bacteria can be altered chemically so that they are harmless. These harmless toxins are called toxoids. Toxoids are used in vaccines to provide protection against some infectious diseases.

(a)Describe the response of the immune system to the injection of a toxoid.[5]
(b)(i)Myasthenia gravis (MG) is described as an autoimmune disease. It is a long-term condition that results from a failure of the immune system. Explain why MG is known as an autoimmune disease.[3]
(b)(ii)Suggest why MG is a long-term condition.[2]

Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[5] five from: A 'cells' for 'lymphocytes' throughout 1 ref. to antigen presentation; A macrophage presents antigen 2 recognition / binding (in context of B-, or T-lymphocytes); A immunoglobulin / antibody on surface of B-lymphocytes e.g. cell surface receptor specific to toxoid / clonal selection 3 T-lymphocytes or B-lymphocytes, divide by mitosis / clonal expansion ; 4 (some) plasma cells, formed / AW ; R if in context of T-lymphocytes 5 antibody molecules / antibodies, secreted / produced / released ; R if in context of T-lymphocytes 6 T helper cells secrete, cytokines / interleukins; e.g. to stimulate, humoral response / B-lymphocytes / to stimulate macrophages / angry macrophages 7 ref. to action of cytokines / interleukins; 8 formation of memory cells; only in context of B-/T-lymphocyte activation
[3] three from: 1 failure to distinguish between self and non-self / AW; 2 specific B-,lymphocytes / cells, are not destroyed (during development); 3 antibodies are produced against, cell surface / ACh, receptors on, muscle cell / neurones / nerve cells ; 4 antibodies, bind to / block, receptors; A antibody-antigen, binding / complex if receptors already given 5 AVP; any detail related to the disease impulses are not transmitted across synapse / neurones cannot send impulses e.g. (chemical) transmitter molecules / acetylcholine, cannot bind (to receptor) ref. to causing, muscle weakness / muscle fatigue ref. to neuromuscular junction does not function
[2] two from: so immune response, occurs all the time / recurs ; (defective) B-lymphocytes still being produced / AW ; IT (helper) cells A not destroyed / remain plasma cells / B-lymphocytes, continue to release antibodies against receptors ; memory cells / immunological memory, present ; receptors are always present (on neurones / nerve cells); AVP; e.g. no cure