Antibodies are produced by plasma cells. Fig. 6.1 shows antigens bound to antigen-binding sites of an antibody molecule. [Figure 6.1] It is estimated that the immune system of each person can make enough antibodies to bind to over 10¹² different antigens. When plasma cells make antibody molecules they combine the polypeptides produced by the expression of genes for heavy chains and the genes for light chains. Research has shown that producing this very large number of antibodies is only possible by modifying the primary transcripts of the genes that code for heavy chains and the genes that code for light chains.

Mark scheme

Marking points from the official mark scheme (May/June 2024). The mark or mark type is shown in brackets.

1. [2] any two from:
2. [2] 1 (antigen-binding site / variable region) has a shape that is complementary to antigen; A tertiary / quaternary, structure
3. [2] 2 (antigen-binding site / variable region has) specific, sequence(s) of amino acids / primary structure(s);
4. [2] 3 idea that different amino acids have different, R-groups / side chains, so give different, tertiary structures / shapes (for binding to antigens);
5. [2] 4 AVP; e.g. correct ref. to epitope(s)
6. [1] idea that allows flexibility for binding (to antigens) ; A at different angles for flexibility / allows variable region(s) to move
7. [1] any one from:
8. [1] idea that easier for macrophage to engulf, antibodies that have bound antigens / antibody-antigen complexes ;
9. [1] facilitates / AW, destruction of pathogens 'marked' by antibodies ; A ref. to opsonisation I stimulates phagocytosis of the pathogen, unqualified
10. [2] any two from:
11. [2] 1 removal of introns (from primary transcript);
12. [2] 2 (after removal of introns) exons, are joined together, in different, sequences / combination(s) ; A alternative splicing
13. [2] 3 not all the exons are used (in making the polypeptides) ; I capping and poly-A tails