Immunity: Question 7

Worked solution

Part (a): Why so many types of B-lymphocyte already exist

Long before a person meets a particular pathogen, their body has already produced a vast range of different B-lymphocytes. Each one carries a receptor on its cell-surface membrane with a slightly different shape, generated more or less randomly during the lymphocytes’ development. Because this diversity is created in advance, covering an enormous number of possible shapes, it is extremely likely that at least one existing B-lymphocyte will, by chance, have a receptor complementary to the antigen of almost any new pathogen the body later encounters.

Part (b): Clonal selection and clonal expansion

When the new pathogen enters the body, its antigen only binds to the receptor of the one type of B-lymphocyte (out of the huge variety present) whose receptor shape is complementary to it. This process of “choosing” the correct B-lymphocyte by antigen binding is called clonal selection. Full activation of the selected B-lymphocyte is usually also helped by chemical signals from an activated T-helper cell that has recognised the same antigen.

Once selected, this B-lymphocyte divides repeatedly by mitosis, producing a large number of genetically identical daughter cells, all carrying the same antigen receptor. This increase in the number of cells belonging to the selected clone is called clonal expansion. It ensures that, instead of relying on just one or a few B-lymphocytes, the body rapidly builds up a large population capable of responding to this particular pathogen.

Part (c): What the expanded clone differentiates into

The cells produced by clonal expansion differentiate into two main types:

• Plasma cells: these secrete large quantities of soluble antibody, specific to the pathogen’s antigen, into the blood plasma and tissue fluid, where it can act against the pathogen (for example, by agglutination, opsonisation or neutralisation).
• Memory cells: these do not secrete antibody straight away. Instead they remain circulating in the body, often for years, ready to respond very quickly (proliferating and differentiating into plasma cells with a much shorter lag period) if the same antigen is encountered again.

Final answers

• (a) A huge variety of B-lymphocytes with different receptor shapes already exists before infection, making it likely one is complementary to any new antigen.
• (b) Clonal selection: the antigen binds only the B-lymphocyte with a complementary receptor. Clonal expansion: that B-lymphocyte then divides by mitosis to form a large clone of identical cells.
• (c) Plasma cells (secrete antibody against the antigen) and memory cells (persist to give a faster secondary response later).