B lymphocytes and the humoral responseAQA A-Level Biology: Revision notes
Section 1
The humoral response
The humoral response involves B lymphocytes (B cells) and antibodies in body fluids such as blood plasma and tissue fluid. Each B cell has antibody molecules on its surface as receptors, with a specific shape that binds to one antigen. It deals mainly with pathogens outside cells.
Humoral means 'of the body fluids'. Antibodies work in blood and tissue fluid, outside cells.
Section 2
Clonal selection and the release of antibodies
- A B cell whose surface antibody is complementary to the antigen on a pathogen binds to it. This is clonal selection: only the matching B cell is chosen.
- Cytokines from helper T cells stimulate the selected B cell.
- The B cell divides by mitosis (clonal expansion) and differentiates into plasma cells and memory cells.
- Plasma cells secrete large quantities of one type of antibody, the same specificity as the original B cell. Because they come from one clone, these are monoclonal antibodies.
Memory cells do not secrete antibodies. Plasma cells do.
Section 3
Antibody structure
An antibody is a protein (an immunoglobulin) made by plasma cells that binds to a specific antigen.
- Four polypeptide chains: two identical long (heavy) and two identical short (light) chains, held together by disulfide bridges.
- Each chain has a variable region, whose tertiary structure is complementary to one antigen, making the antigen-binding site. Each antibody has two of these.
- The constant region is the same within a class of antibody.
- A flexible hinge region lets the binding sites move to bind antigens.
The antibody and antigen form an antigen-antibody complex.
If asked why an antibody is specific, say the variable region has a tertiary structure complementary to the antigen.
Section 4
Destroying the antigen
Antibodies lead to destruction of bacterial cells by:
- Agglutination: each antibody has two binding sites, so it binds to antigens on different bacteria and clumps them together.
- Phagocytosis: phagocytes engulf the clumped bacteria and destroy them.
Clumping makes it easier for phagocytes to engulf many bacteria at once.
Section 5
Primary and secondary responses
In the primary response to a first exposure, few B cells have a matching receptor. Selection, division and differentiation take time, so antibody appears slowly and in modest amounts, and symptoms can develop.
The memory cells formed live a long time. On a second exposure to the same antigen, they divide rapidly to produce many plasma cells. This secondary response is faster and produces more antibody, so the pathogen is destroyed before symptoms develop.
A pathogen with a different antigen is not recognised by the memory cells, so it triggers a new primary response.
Do not say memory cells make antibodies directly. They divide to form plasma cells, which make the antibodies.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on B lymphocytes and the humoral response
- A patient was infected with a bacterium for the first time. A laboratory measured the concentration of a specific antibody against the bacterium in samples of her blood. No antibody was detectable for the first few days; it then rose and peaked after about two weeks.Explain why an antibody against this bacterium does not bind to the antigens of other pathogens.2 marks
- A biochemist analysed an antibody molecule from a patient's blood. The molecule consisted of four polypeptide chains: two identical long chains and two identical short chains. The chains were held together by bonds between sulfur atoms of cysteine residues, and each molecule had two identical sites that bound to the antigen.Explain how agglutination of bacteria by antibodies helps to destroy them.2 marks
- A child was exposed to the measles virus for the first time at the age of two. The concentration of anti-measles antibody in the blood rose slowly and peaked at about 14 days, and the child had a fever and a rash. At the age of six she was exposed to the virus again. Antibody appeared in her blood within two days, reached a concentration about 100 times higher, and she showed no symptoms.Explain the differences between the antibody responses to the first and second exposures to the virus.3 marks
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).