Immunity: Question 10

Worked solution

Part (a): Humoral immunity

Humoral immunity is mainly carried out by B-lymphocytes. When a B-lymphocyte with a complementary receptor is activated (with help from a T-helper cell), it undergoes clonal selection and expansion, and many of the resulting cells differentiate into plasma cells. Plasma cells secrete large amounts of soluble antibody into the blood plasma and tissue fluid (the body’s “humours”, giving humoral immunity its name). These antibodies act on pathogens and toxins that are outside body cells, for example by agglutinating bacteria into clumps, opsonising them to mark them for phagocytosis, or neutralising toxins by binding to their active regions.

Part (b): Cell-mediated immunity

Cell-mediated immunity is mainly carried out by T-lymphocytes. T-killer (cytotoxic) cells recognise the body’s own cells that have become infected, because infected cells display fragments of the pathogen’s antigen on their surface. Once a T-killer cell binds to such an infected cell, it destroys that cell directly. T-helper cells support this response by releasing chemical signals (cytokines) that help activate T-killer cells, B-lymphocytes and phagocytes, though they do not destroy infected cells themselves.

Part (c): Why cell-mediated immunity matters for an intracellular virus

Antibodies are relatively large, soluble protein molecules that circulate in the blood plasma and tissue fluid; they cannot cross the cell-surface membrane to reach a virus that has entered and is replicating inside a body cell. This means humoral immunity alone cannot deal with the virus once it is hidden inside a cell.

Cell-mediated immunity solves this problem differently: rather than needing to reach the virus itself, T-killer cells recognise the infected cell (via the viral antigen it displays on its surface) and destroy the entire infected cell. Destroying the infected cell also destroys the virus particles replicating inside it, before they can be released and go on to infect further body cells, limiting the spread of the infection.

Final answers

• (a) B-lymphocytes → plasma cells secreting antibodies (agglutination, opsonisation, neutralisation) against pathogens outside cells.
• (b) T-lymphocytes → T-killer cells destroy infected body cells directly; T-helper cells activate other immune cells.
• (c) Antibodies cannot reach a virus inside a body cell, but T-killer cells recognise and destroy the infected cell itself, removing the virus’s site of replication.