The response of T lymphocytes to a foreign antigen (the cellular response).

The idea

T lymphocytes mature in the thymus, each carrying receptors of a single specific tertiary structure. They respond only to antigens presented on a cell surface, targeting virus-infected cells or transplanted tissue. When an antigen-presenting cell displays a foreign antigen, a T cell with a complementary receptor binds to it. This activates the T cell, causing it to divide by mitosis (clonal expansion). The resulting clone is lineage-specific: activated helper T cells release cytokines to stimulate phagocytes and B cell division, while cytotoxic T cells kill infected cells by releasing perforin, making the cell-surface membrane freely permeable. Both lineages also produce memory T cells for a rapid secondary response.

What earns the marks

• A foreign or faulty protein being recognised as an antigen on an antigen-presenting cell.
• A specific T cell with a complementary receptor binding to the presented antigen.
• The activated T cell dividing by mitosis to form a clone (clonal expansion).
• Helper T cells releasing cytokines that stimulate phagocytes and B cell division.
• Cytotoxic T cells releasing perforin to make the infected cell's membrane freely permeable.

Common mistakes and how to put them right

• Stating that T cells produce antibodies.

  Put it right: Only plasma cells derived from B cells secrete antibodies.

• Claiming that helper T cells stimulate clonal selection of B cells.

  Put it right: Antigens cause selection, while helper T cells stimulate clonal expansion.

• Stating that cytotoxic T cells 'kill the cell' without a mechanism.

  Put it right: You must specify that perforin makes the membrane freely permeable.

Exam tips

• Always use 'complementary' when describing the fit between the T cell receptor and the antigen; 'same shape' will not be credited.
• When asked how the immune system destroys abnormal cells, start by stating that the abnormal protein is recognised as a foreign antigen.