Y6 Test and Review
2024
Y6 Test and Review 19
Infectious Disease
25 minutes
3(a)
With reference to Fig.3.1, name the cells and the responses involved in the immune system.
Cell A
Dendritic cell
Cell B
Macrophage
Cell C
Neutrophil
Response I
Innate immune response
Cell D
Helper T cell
Cell E
Cytotoxic T cell
Response II
Cell-mediated response
Cell F
Plasma cell (R: B cell)
[3]
Two correct per mark awarded.
No half marks will be awarded.
Students who get 3 answers correct will get 1 mark.
Students who get 5 answers correct will get 2 marks.
Teacher’s Comments:
• Well-answered question.
• With reference to the following points:
o Response I – phagocytosis was not accepted. While it is true that Cell A, B and
C are all phagocytes, the correct term is “innate immune response”.
o Response II – specific/adaptive immunity was a common incorrect answer.
specific/adaptive immunity was not accepted as the answer needed to be more
specific. Do note that there are two types of specific/adaptive immunity
responses, one is humoral and the other is cell-mediated
(b)
1. The constant region of their heavy chains are different;
A: Different Fragment Crystallisable (Fc)
A: IgG has a gamma chain at its constant region of heavy chain and IgM has a mu
chain
2. Different conformation/shape/amino acid sequences of the constant heavy chains;
3. IgM can exist as a pentamer, having 5 immunoglobulins attached at their heavy chains
while IgG is monomeric;
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Comments:
1. Some students didn’t realise that constant chain alone is insufficient because there is
also the constant light chain.
2. Some students mentioned that the variable region, being variable, is likely to be
different. No! The VDJ (variable) gene segment of the heavy chain remains unchanged
between IgG an IgM. They bind the exact same antigen.
3. IgM and IgG are proteins not DNA.
4. Just for your info, a membrane bound variant of IgM is the B cell receptor which you are
familiar with.(BCR)
(c)
(i)
1. Peak earlier: IgM concentration reaches a peak on day 13/14 while IgG concentration
peaks later on day 21/22;
2. Higher peak: IgG reaches a higher peak than IgM;
3. Longer duration: IgG was above the basal level for a longer period of time from day 12
to day 30 (18 days) while IgM was above the basal level from day 10 to day 17 (7
days);
AVP
Comments:
1. Students need to do two things here. Quote if applicable and use a comparative word
such as higher, longer, earlier, faster etc. We need you to interpret data and numbers
on their own are meaningless without your interpretation. E.g. for 55 and 58. Someone
might view that they are sufficiently similar and not significantly different (e.g. in genetics
or statistics) while someone else may view 58 as an increase over 55. We want your
interpretation in that specific context.
2. Quote figures accurately Use a ruler, measure and convert by proportion.
3. Quite a number of students literally described the two graphs progressing from left to
right. Of course all the plot points are not going to be the same except for the
intersection point(s).Technically every of those points constitute a difference but are
those differences meaningful? Perhaps if it was viewed differently as shown below, you
can pick out more significant differences.
There are however times when we do need to segment the graph and describe each
segment e.g. the temperature response curve in enzyme reactions. Exponential
increase, peak, steep decline due to denaturation of enzymes. There are various ways
to look at a graph but you need to be flexible in how you interpret the graph in a
meaningful manner.
4. Some students didn’t compare point for point and instead give a lengthy description of
graph IgM followed by IgG. We do not match the points for you.
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(ii)
The graph is slightly different from the graph in the notes. Gradient of rise remains
unchanged. Use the context that is given.
Explanation
Observations
1. The lag period is 11/12 days in the
primary immune response while it is
much shorter at 4 days in the
secondary immune response;
2. as the memory B cells take a
shorter time/easier for
reactivation/differentiate into
plasma cells which go on to
produce IgG;
or reverse argument (ORA)
3. The IgG concentration reaches a
much higher level for the secondary
response;
A: stronger immune response;
ORA;
4. Pool of memory B cells differentiate
to form more plasma cells which
produce more antibodies in the
secondary response;
ORA;
5. The IgG concentration in the
secondary response is sustained for a
longer period of time;
ORA;
6. As the long lived memory B cells
can continue to proliferate and
differentiate into plasma cells /
more plasma cells; AVP
Comments:
1. Again students need to quote accurately and have a comparative descriptor.
2. Some students claim that the secondary response has no lag period. Lag period is the
period (post exposure) where antibody concentration has not risen. No label doesn’t
mean no lag period.
3. Many students were not explaining why the immune response was faster, merely
repeating that it was so. When explaining the reduced lag period, focus on the reduction
in time needed for reactivation. There is still a need for memory B cells to process
antigen, present antigen, interact with T helper cell, proliferate and differentiate i.e. the
same steps that are needed for primary immune response. These steps are needed to
ensure the specificity of the response. The process of reactivation simply takes place
faster.
4. Again students merely repeat that more antibodies are produced without offering a
reason why. When explaining the increased concentration of antibodies, don’t talk about
the speed of reactivation but focus on how more antibodies can be produced. More
plasma cells → more antibodies. Each plasma cell has a limited capacity to upregulate
gene expression. Each plasma cell cannot just increases its antibody production.
5. Memory B cells do not produce antibodies, only plasma cells do.
6. Some students explained that the reason for the sustained and high level of antibody
concentration was because such levels was necessary to eliminate the pathogen. This
is not true as the first antigen exposure showed that the lower levels was sufficient to
eliminate the pathogen. The secondary response is an overkill so you don’t even fall
sick. So you need to argue from the process angle why the levels are that high and
sustained.
7. We taught you “plasma cells”. Where did “plasma B cells” come from?
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(d)
1. Class switching*
2. where DNA rearrangement occurs at the constant gene segment of the Ig heavy chain
gene locus;
Comments:
1. Most could identify class switching but if any other process was included, no mark was
given. We don’t choose correct answers for you.
2. To get the second mark, you need to mention that this occurs at the DNA level. And it
involves the constant, heavy chain.
[Total: 12]
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