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2.5.7Cell Structure

Plasma and Memory Cells — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 35 of 47 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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2.5.7 Plasma and Memory Cells — AQA A Level Biology 7402 notes: Compare plasma cells and memory cells, and explain how they produce primary and secondary immune responses and long-term immunity.When a B cell is activated, it divides to make a clone and then the cells differentiat…

  • rough endoplasmic reticulum
  • Golgi apparatus
  • mitochondria
  • longer lasting
  • antigenic variation
  • Exam-style question 1

Section 1 of 9Two products of clonal expansion

When a B cell is activated, it divides to make a clone and then the cells differentiate into two functional types: plasma cells and memory cells.

Two products of clonal expansion
A similar division occurs in T cells, which form effector cells (cytotoxic and helper T cells) and T memory cells.
The key principle is that the immune response must do two things: deal with the current infection and prepare the body for the next one.

Plasma cells are the effector cells of the humoral response. They live for only a few days to weeks, but each can secrete thousands of antibody molecules per second. Memory cells are long-lived, sometimes for decades, and do not secrete antibodies; they remain ready in the blood and lymphoid tissue.

Section 2 of 9Structure and function of plasma cells

Plasma cells are adapted for protein secretion.

Structure and function of plasma cells
Rough endoplasmic reticulum
They have an extensive rough endoplasmic reticulum where antibody polypeptides are synthesised on ribosomes, a prominent Golgi apparatus which modifies the proteins (adds carbohydrate to form glycoproteins) and packages them into vesicles, and many mitochondria to provide ATP.
The vesicles fuse with the cell-surface membrane and release antibodies by exocytosis.
They typically have a large amount of cytoplasm and an off-centre nucleus.

Since the antibodies secreted by a plasma cell clone are all identical and have the same antigen-binding site, they are specific to the antigen that triggered the clone.

Section 3 of 9Memory cells and the secondary response

Memory cells carry the same receptor as the original activated cell. When the same antigen is met again, they are quickly selected, divide by mitosis, and differentiate into plasma cells (or cytotoxic T cells). Because there are many more cells with the right receptor than at the first exposure, the response is:

faster
faster antibody can be detected within hours or 1-2 days rather than 5-10 days;
stronger
stronger antibody concentration reaches a much higher peak;
longer lasting
longer lasting antibodies remain in the blood for longer.

As a result, the pathogen is destroyed before it can cause symptoms. This is why people rarely catch diseases such as chickenpox or measles twice.

FeaturePlasma cellMemory cell
FunctionSecretes antibodiesProvides memory for future response
LifespanShortLong
Rough ERExtensiveLittle
DividesNo (terminal differentiation)Yes, on re-exposure

Section 4 of 9Why some diseases can recur

Memory cells do not always give lasting protection.

Antigenic variation
Some pathogens undergo antigenic variation: mutations change the shape of their surface antigens, so memory cells for the old antigen cannot bind to the new variant.
Influenza viruses change each year, so people can catch flu repeatedly and the vaccine has to be redesigned.
Some pathogens, such as the malarial parasite, change antigens during the infection.
Also, if there are many different strains (as with the common cold viruses), a person will not have memory cells for the new one.

The idea of immunological memory is the basis of vaccination, in which a harmless form of the antigen is given to produce memory cells without the person suffering the disease.

Section 5 of 9Reading antibody graphs

You will often meet a graph of antibody concentration over time for two exposures.

1
For the first exposure, look for the lag phase (no antibody), a slow rise to a low peak, and a gradual fall.
2
At the second exposure look for a shorter lag, a steeper rise, a higher peak (perhaps ten times higher) and slower decline.
3
Quote values and times, and explain differences using memory cells.
4
If a different antigen is used at the second exposure, the response will resemble a primary response again because no memory cells exist for it.

Section 6 of 9How memory is used in medicine

Vaccines exploit memory cells.

1
A vaccine contains an antigen from the pathogen, and the first dose produces a primary response and memory cells.
2
A booster dose given weeks or months later acts as a second exposure, producing a larger and longer-lasting secondary response and a higher concentration of memory cells.
3
Booster doses are therefore used for tetanus, whooping cough and many other vaccines.
4
A test for antibodies in the blood (serology) can show whether a person has been exposed to a pathogen: the presence of antibodies indicates a previous infection or vaccination.
5
In contrast, giving a person ready-made antibodies, such as antivenom, provides passive protection which does not result in memory cells, and so protection is short-lived.
6
The detailed comparison of active and passive immunity is covered in the next topic.

Section 7 of 9Exam-style questions

  1. Describe the roles of plasma cells and memory cells. [3]3 marks

  2. Explain how the structure of a plasma cell is adapted to its function. [3]3 marks

  3. The antibody concentration after a first infection peaked at 20 units on day 14. After a second infection by the same pathogen it peaked at 180 units on day 4. Calculate how many times higher the second peak was and explain the difference. [3]3 marks

Section 8 of 9Key terms

TermMeaning
Plasma cellDifferentiated B cell that secretes antibodies
Memory cellLong-lived lymphocyte that gives a rapid response on re-exposure
Primary responseSlow, weak response to first exposure to an antigen
Secondary responseFast, strong response to re-exposure to an antigen
Immunological memoryAbility to respond faster to a previously met antigen
Antigenic variationChange in antigens of a pathogen that avoids memory cells
ExocytosisRelease of substances from vesicles that fuse with the cell-surface membrane

Section 9 of 9Exam technique

1
Saying memory cells secrete antibodies - they differentiate into plasma cells when needed.
2
Writing that memory cells make the body immune forever - antigenic variation can bypass memory.
3
Forgetting that plasma cells are short-lived.
4
Stating that the antibody concentration rises faster only because the pathogen is more numerous.
5
Confusing active immunity (memory cells) with passive immunity (no memory).

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