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The immune responseEdexcel A-Level Biology B: Revision notes

Section 1

Phagocytes and lymphocytes

The immune system uses phagocytes and lymphocytes.

  • Neutrophils are the first phagocytes to arrive at a site of infection, in large numbers. They engulf pathogens by phagocytosis, and many die and form pus.
  • Macrophages are longer-lived phagocytes. They engulf and digest pathogens, and then display the pathogen's antigens on their surface as antigen-presenting cells.
  • Lymphocytes are the cells of the specific immune response. B cells make antibodies and T cells coordinate and carry out the response.

An antigen is a molecule, usually a protein, that triggers an immune response.

Key termsneutrophilmacrophagelymphocyteantigen

Section 2

Phagocytosis and antigen presentation

In phagocytosis a phagocyte engulfs a pathogen into a vesicle (phagosome). The vesicle fuses with a lysosome and the enzymes in the lysosome digest the pathogen.

A macrophage then places antigens from the pathogen on its surface membrane. It is now an antigen-presenting cell, and the antigens are recognised by T helper cells, which links the non-specific response to the specific immune response.

Key termsphagocytosislysosomeantigen-presenting cell
Exam tip

Macrophages present antigens. Plasma cells make antibodies. Do not mix these up.

Section 3

The humoral response

The humoral response involves antibodies in body fluids, made by B cells.

  1. A macrophage (antigen-presenting cell) displays the antigen. A T helper cell with a complementary receptor binds to it and is activated.
  2. The T helper cell releases cytokines.
  3. A B cell with a complementary antibody on its surface binds to the antigen. The cytokines stimulate it. This is clonal selection, as only the B cell with the right antibody is chosen.
  4. The selected B cell divides by mitosis (clonal expansion) to form a clone of identical cells.
  5. Many become plasma cells, which secrete large amounts of antibody. Some become B memory cells.

Antibodies bind to the antigens of the pathogen, which helps to destroy it.

Key termshumoral responsecytokineclonal selectionclonal expansionplasma cellantibody

Section 4

The cell-mediated response

The cell-mediated response destroys body cells that are infected with a virus (or are abnormal) and uses T cells.

  1. An antigen-presenting cell, such as an infected cell or a macrophage, displays the antigen on its surface.
  2. A T helper cell with a complementary receptor binds to it and is activated. It releases cytokines.
  3. The cytokines stimulate T killer cells with a complementary receptor to divide and become active. They also stimulate macrophages.
  4. A T killer cell binds to an infected cell and releases toxic substances, such as perforin, which cause it to die.

Antibodies cannot enter cells, so this response is needed against pathogens inside cells.

Key termscell-mediated responseT helper cellT killer cell
Common mistake

Do not say T cells make antibodies. Only plasma cells, which come from B cells, make antibodies.

Section 5

Memory cells and the secondary response

After the first (primary) response, some B cells and T cells become memory cells. They stay in the body for a long time.

On a second exposure to the same antigen, memory cells recognise it quickly. B memory cells divide rapidly to form many plasma cells, and T memory cells form many T killer cells. The secondary response is therefore faster, larger and longer lasting, and the pathogen is destroyed before it causes symptoms. This is the basis of vaccination.

Key termsmemory cellsecondary response

Must Know

  • Neutrophils arrive first; macrophages engulf pathogens and present antigens
  • Humoral response: antigen presentation, T helper cell, cytokines, B cell clonal selection, clonal expansion, plasma cells making antibodies
  • Cell-mediated response: T helper cells and cytokines activate T killer cells, which destroy infected cells
  • T and B memory cells give a faster, larger secondary response

That's the notes covered.

Carry on to the next subtopic.

Exam questions on The immune response

  1. A patient has a cut that becomes infected with bacteria. Within a few hours the area is swollen and red, and large numbers of phagocytes are seen in the tissue. Many of the first phagocytes to arrive die at the site and form pus, while other phagocytes remain active for longer.
    Explain how a phagocyte destroys a bacterium after engulfing it.2 marks
  2. Researchers infect a mouse with a virus. A few days later they find that a small number of B cells, each with surface antibodies that bind to a viral protein, have divided many times to form a large number of identical cells. Some of these cells are secreting large amounts of antibody into the blood.
    Explain why only a small number of B cells divided in response to the viral protein.2 marks
  3. A virus enters the liver cells of a patient and replicates inside them. Fragments of viral protein are displayed on the surface membranes of the infected cells. The patient's blood contains antibodies against the virus, but the virus inside the liver cells is still not cleared until other cells of the immune system respond.
    Describe how T helper cells are activated in the cell-mediated response and how they activate T killer cells.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).