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6.2.6Topic 6 · Immunity, Infection and Forensics

B Cell Response — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 9 parts Lesson 14 of 19 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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6.2.6 B Cell Response — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the humoral immune response, including clonal selection of B cells, the formation of plasma and memory cells, and the role of T helper cells.B lymphocytes (B cells) are produced and mature in the bone m…

  • humoral immunity
  • Activation
  • Clonal expansion
  • Differentiation
  • plasma cells
  • memory B cells

Section 1 of 9B cells and humoral immunity

B lymphocytes (B cells) are produced and mature in the bone marrow.

Each B cell has antibody molecules of one type embedded in its surface membrane as receptors.
Humoral immunity
The B cell response is called humoral immunity because the effectors, antibodies, are carried in the body fluids (the blood and lymph, formerly called 'humours').
It is most effective against pathogens and toxins outside cells, such as bacteria and free viruses.

The human body has millions of different B cells, each with a unique receptor. The diversity arises from random rearrangement of gene segments during B cell development, so almost any antigen can be matched by at least one B cell.

Section 2 of 9Step-by-step response

1
Binding
Binding a pathogen with antigen enters the body. The antigen binds to a B cell with a complementary surface antibody (receptor). Only the B cell with the matching receptor is selected: this is clonal selection.
2
Activation
Activation the B cell takes in the antigen by endocytosis, processes it and displays it on its surface.
  • A T helper cell with a receptor that fits the same antigen binds to it and releases cytokines, which activate the B cell.
  • Full activation of most B cells requires this T cell help.
3
Clonal expansion
Clonal expansion the activated B cell divides repeatedly by mitosis, producing a clone of genetically identical cells.
4
Differentiation
Differentiation most clone cells become plasma cells, which have abundant rough endoplasmic reticulum, Golgi apparatus and mitochondria to make and secrete large quantities of antibody (several thousand molecules per second). Others become memory B cells.
5
Action
Action antibodies bind the antigen on the pathogen, causing agglutination, opsonisation and neutralisation, so phagocytes remove it.

Section 3 of 9Memory and the secondary response

Memory B cells live for years and carry the same surface antibody.

Memory and the secondary response
On re-exposure to the same antigen they are quickly activated, divide and differentiate into plasma cells.
This gives a larger and faster secondary response, with antibody concentration rising within one or two days and reaching a higher peak, and it often prevents symptoms.
Memory is specific: immunity to one pathogen gives no protection against a different one, and for pathogens that change their antigens, such as influenza, the memory cells may not recognise the new form.
FeaturePrimary responseSecondary response
Lag timeSeveral daysHours to 1 to 2 days
Peak antibody concentrationLowerMuch higher
DurationShortLonger
Cells respondingNaive B cellsMemory B cells

Section 4 of 9Interpreting antibody graphs

Graphs of antibody concentration against time after infection or vaccination are common.

1
For the first exposure, give the lag period before antibodies appear (the time needed for clonal selection and expansion), the peak concentration and the fall.
2
For a second exposure, point out the shorter lag, higher peak and slower decline, and explain these changes by memory cells.
3
A booster vaccination uses this principle to raise antibody concentration and the number of memory cells.
4
When there is a different antigen on second exposure, the response resembles a primary response.

Section 5 of 9Applications

Monoclonal antibodies, made from a clone of a single B cell fused with a tumour cell to give a hybridoma, are used in pregnancy tests, diagnosis and treatment of disease. Understanding the humoral response explains how vaccines work and why some people with defective B cells are susceptible to repeated bacterial infections.

Section 6 of 9Hybridomas and clinical use

To make monoclonal antibodies, a mouse is immunised with an antigen, and B cells that make the specific antibody are collected from the spleen.

Hybridoma
These cells cannot divide indefinitely, so each is fused with a cancerous myeloma cell, forming a hybridoma that can both divide continuously and produce the antibody.
Hybridomas are screened, the best clone is grown in large culture, and the antibodies are collected and purified.
Therapeutic monoclonal antibodies are used to target cancer cells by binding to antigens that are abundant on tumour cells, to deliver drugs, or to block signals that stimulate growth.
Early versions made from mouse cells could cause immune reactions in patients, so they have been modified to be more human-like.

Remember that the specificity of antibodies is the property that makes them useful in diagnosis and treatment, and that ethical issues include the use of animals in the production process.

Section 7 of 9Exam-style questions

  1. Describe how a B cell responds to a bacterium when it enters the body for the first time. [5]5 marks

  2. Explain why plasma cells have a large amount of rough endoplasmic reticulum and many mitochondria. [3]3 marks

  3. The antibody concentration reached 10 units 14 days after the first exposure and 80 units 5 days after the second. Compare the responses. [3]3 marks

Section 8 of 9Key terms

TermMeaning
B cellA lymphocyte that matures in bone marrow and gives rise to antibody-secreting cells.
Plasma cellAn activated B cell that secretes large amounts of antibody.
Humoral immunityImmunity mediated by antibodies in body fluids.
Clonal selectionSelection of the B or T cell whose receptor fits the antigen.
Memory B cellLong-lived cell that gives a faster response on re-exposure.
CytokineSignalling protein from T helper cells.
Monoclonal antibodyIdentical antibodies from a single clone of B cells.

Section 9 of 9Exam technique

1
Saying that B cells attack pathogens. They produce antibodies via plasma cells.
2
Forgetting T helper cell involvement in activation.
3
Saying memory cells secrete antibodies. They are ready to divide into plasma cells when re-exposed.
4
Writing that the immune system chooses which antibodies to make. The existing B cells are selected by antigen.

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