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How does the adaptive immune system mount a specific response to a particular pathogen using lymphocytes?

Describe the adaptive immune response, including the roles of T and B lymphocytes in the cell-mediated and humoral responses

A focused answer to the H2 Biology Infectious Disease and Immunity outcome on adaptive immunity. Antigens and antigen presentation, the cell-mediated response by T lymphocytes, the humoral response by B lymphocytes, clonal selection, and the production of plasma and memory cells.

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  3. Examples in context
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What this dot point is asking

SEAB wants you to describe the adaptive (specific) immune response: how antigens are recognised, the roles of T lymphocytes (cell-mediated response) and B lymphocytes (humoral response), and the processes of clonal selection and expansion that produce effector and memory cells. This builds on phagocytosis and antigen presentation and leads into antibodies and memory.

The answer

Antigens and recognition

An antigen is a molecule (often a surface protein) recognised as foreign by the immune system. Each lymphocyte has a specific receptor; the body has a huge variety of lymphocytes, so there is at least one able to recognise almost any antigen.

Antigen presentation

After a phagocyte (such as a macrophage) engulfs a pathogen, it displays the pathogen's antigens on its surface, becoming an antigen-presenting cell. This activates the adaptive response.

The cell-mediated response (T lymphocytes)

A helper T lymphocyte with a complementary receptor binds the presented antigen and is activated. It then:

  • divides and releases signalling molecules that stimulate other cells,
  • activates cytotoxic T lymphocytes, which destroy the body's own infected cells, and
  • stimulates the appropriate B lymphocytes.

The humoral response (B lymphocytes)

A B lymphocyte whose receptor (antibody) is complementary to the antigen is selected (clonal selection) and, helped by the helper T cell, activated. It divides by mitosis (clonal expansion) into:

  • plasma cells, which secrete large amounts of specific antibody, and
  • memory B cells, which persist for a faster response if the same pathogen returns.

Examples in context

Example 1. Why the first infection takes longer. On first exposure, the body must select and expand the few lymphocytes specific to the new pathogen, so the response is slow and symptoms may develop. This delay is exactly what immunological memory removes on a second exposure.

Example 2. Helper T cells as a hub. Because helper T cells activate both cytotoxic T cells and B cells, their loss (as in some viral infections that destroy helper T cells) cripples the whole adaptive response, leaving the body open to many infections. This shows how central coordination is to immunity.

Try this

Q1. State what is meant by an antigen. [1 mark]

  • Cue. A molecule (often a surface protein) recognised as foreign by the immune system and able to trigger an immune response.

Q2. Describe the role of plasma cells in the humoral response. [2 marks]

  • Cue. Plasma cells are formed by the division of activated B cells and secrete large amounts of a specific antibody complementary to the antigen.

Q3. Explain the role of helper T lymphocytes in the adaptive response. [2 marks]

  • Cue. They bind presented antigen, are activated, and then stimulate cytotoxic T cells and B cells, coordinating both arms of the adaptive response.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original6 marksDescribe the roles of T lymphocytes and B lymphocytes in the adaptive immune response to a bacterial infection.
Show worked answer →

Examiners want antigen presentation, T cell roles, and B cell roles.

An antigen-presenting cell, such as a macrophage that has engulfed the bacterium, displays the bacterial antigens on its surface. A helper T lymphocyte with a complementary receptor binds the displayed antigen and is activated.

The activated helper T lymphocyte divides and releases signalling molecules that stimulate other cells. It activates cytotoxic T lymphocytes, which destroy infected body cells, and it stimulates B lymphocytes.

A B lymphocyte with a receptor (antibody) complementary to the antigen is selected and, helped by the helper T cell, is activated. It divides by mitosis (clonal expansion) to form plasma cells, which secrete large amounts of specific antibody against the bacterium, and memory B cells, which remain for a faster future response.

Markers reward antigen presentation activating a helper T cell, the helper T cell stimulating cytotoxic T cells and B cells, the selection and division of the specific B cell, and the production of plasma cells (antibodies) and memory cells.

Original4 marksExplain what is meant by clonal selection and clonal expansion in the adaptive immune response.
Show worked answer →

The answer should define both terms and link them.

The body has a huge variety of lymphocytes, each with a different specific receptor, so that there is at least one lymphocyte able to recognise almost any antigen. Clonal selection is the process by which an antigen selects and activates only the few lymphocytes whose receptors are complementary to that antigen, while the others remain inactive.

Clonal expansion is the rapid division of the selected lymphocytes by mitosis to produce a large clone of identical cells, all specific to the same antigen. These then differentiate into effector cells (such as plasma cells or cytotoxic T cells) and memory cells.

Markers reward the existence of a varied lymphocyte population, the selection of only the complementary lymphocytes by the antigen, and their division to form a clone of identical specific cells.

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