Page 223 - SUBSEC October 2017_Neat

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02207020/CAPE/SPEC/KMS 2017
BIOLOGY
UNIT 1 – PAPER 02
KEY AND MARK SCHEME

Question 3 cont’d

(ii) B-lymphocytes

1. B-cells originate in bone marrow from stem cells by mitosis.
2. Mature in bone marrow.
3. Genes are rearranged to give many (10 million) varying
protein codes.
4. These variants code for different antibody surface
receptors.
5. Each B-cell is programmed to express just one type of surface
receptor during maturation.
6. The range of B-cells will have an immense number of different
receptors.
7. Hence, there will be a B-cell with receptor for any/every
antigen.

5 points – 3 marks
3-4 points – 2 marks
1-2 points – 1 mark [3]

T-lymphocytes

1. Form in bone marrow from stem cells by mitosis.
2. Migrate to thymus.
3. Differentiate genotype to form thousands of variants.
4. Variant genes code for specific surface receptors.
5. T-cells have a wide range of individual surface receptors.
6. T-cells have the body’s own antigens presented to them by
MHC.
7. If a T-cell binds with and reacts with one of the body’s own
protein it is killed.
8. This prevents having T-cells that attack the body’s own
cells, i.e. an autoimmune attack in the body by its own.

5 points – 3 marks
3-4 points – 2 marks
1-2 points – 1 mark [3]

(c) Active immunity

1. The immunity that results from the production of antibodies by the
immune system in response to the presence of an antigen.
2. The result of the response of the body to an antigen/infection,
which has long-term effects.
3. Immunity in an organism resulting from its own production of
antibodies or lymphocytes.

Any one = 1 mark