The role of antigen-presenting cells in the cellular response.

The idea

An antigen-presenting cell displays antigen on its cell-surface membrane, where a T cell with a complementary receptor can bind it. Sources include phagocytes that have hydrolysed a pathogen and moved its antigens to the surface; body cells infected by a virus, which display viral antigens produced inside them; and B cells that have taken in antigen by endocytosis and processed it. Transplanted cells display foreign donor antigens, while cancer cells display abnormal self-antigens. Presentation increases the probability that a complementary T cell meets its antigen: each T cell recognises only one antigen, cells carrying that receptor are rare, and free antigen in lymph is easily missed. Display on a cell surface in lymph nodes, through which lymphocytes circulate, raises the chance of contact. After binding, the helper T cell is activated and divides by mitosis to form a clone.

What earns the marks

• Define an antigen-presenting cell as a cell displaying antigen on its cell-surface membrane, and note that the displayed antigen may be foreign or an abnormal self-antigen.
• Name at least two sources and state how each acquires its antigen: a phagocyte after hydrolysing a pathogen, a virus-infected body cell, or a B cell that has taken in antigen by endocytosis.
• State that a T cell with a complementary receptor binds to the presented antigen.
• Explain that presentation increases the chance of a lymphocyte meeting its specific antigen, because each T cell recognises only one antigen and such cells are rare.
• State that the activated helper T cell divides by mitosis to form a clone.
• Distinguish transplanted cells, which present foreign donor antigens, from cancer cells, which present abnormal self-antigens.

Common mistakes and how to put them right

• Assuming only phagocytes present antigens, and so losing the marks for infected body cells and B cells.

  Put it right: Name at least two sources and state how each acquires its antigen.

• Saying the antigen-presenting cell 'gives' the antigen to the T cell rather than displaying it for binding.

  Put it right: The antigen is displayed on the membrane and the T cell's receptor binds to it.

• Confusing antigen presentation with antibody production.

  Put it right: Presentation is display of antigen; antibody production is secretion by plasma cells.

• Describing antigen-presenting cells as a type of lymphocyte.

  Put it right: Macrophages and infected body cells are not lymphocytes, though B cells are.

• Describing the display of antigen but never stating the advantage it brings, which is where the explain mark sits.

  Put it right: Finish with the increased probability of a complementary T cell meeting the antigen.

• Stating that transplanted cells display their own abnormal antigens.

  Put it right: Transplanted cells carry foreign donor antigens, whereas cancer cells display abnormal self-antigens.

Exam tips

• Finish an 'explain the role' answer with the consequence, activation of the T cell and clonal expansion, not just with the display of antigen.
• When asked to give cell types that stimulate an immune response, antigen-presenting cells such as macrophages and infected body cells are valid examples; check the wording of the question before committing.