Explain the body’s response to immunisation using an inactive form of a…

The idea

Immunisation with an inactive (killed) pathogen exposes the immune system to antigens without causing the disease, because the pathogen cannot reproduce inside the body. After the injection, a white blood cell called a lymphocyte recognises the foreign antigen. The lymphocyte divides to form clones, some of which secrete antibodies that bind to the antigen, and some of which remain as memory cells. Antibody binding helps destroy or neutralise the pathogen. If the live pathogen later enters the body, memory cells respond faster and produce a larger antibody response, so the person is protected. A multiple-choice question may ask which cell type responds, what the inactive pathogen provides, or why memory cells give protection.

What earns the marks

• Inactive pathogen still carries antigens that the immune system recognises as foreign.
• A lymphocyte (white blood cell) recognises the antigen and divides to form clones.
• Some cloned cells secrete antibodies that bind to and help destroy or neutralise the pathogen.
• Some cloned cells become memory cells that remain in the body.
• On later exposure to the live pathogen, memory cells give a faster and larger antibody response, providing protection.
• The inactive pathogen cannot reproduce, so it does not cause the disease itself.

Common mistakes and how to put them right

• Thinking the vaccine directly kills pathogens in the body.

  Put it right: The vaccine stimulates the immune system to make antibodies and memory cells.

• Confusing antibodies with antibiotics.

  Put it right: Antibodies are made by lymphocytes in response to a specific antigen; antibiotics are drugs.

• Believing memory cells are a type of antibody.

  Put it right: Memory cells are lymphocytes that remain and respond quickly on re-exposure.

• Assuming an inactive pathogen can still reproduce and cause disease.

  Put it right: It is killed, so it cannot multiply.

Exam tips

• Read the question stem to see whether it asks about the first response or the later protective response.
• Link each option to a specific cell or molecule: antigen, lymphocyte, antibody, memory cell.
• Eliminate options that confuse antibodies with antibiotics or with the pathogen itself.