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6.2.4Topic 6 · Immunity, Infection and Forensics

Antigens and Antibodies — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 9 parts Lesson 12 of 19 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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6.2.4 Antigens and Antibodies — Edexcel A Level Biology A (SNAB) 9BI0 notes: Define antigen and antibody, describe antibody structure and function, and explain the primary and secondary responses and how vaccination gives immunity.An antigen is a molecule, usually a protein or gl…

  • two heavy chains and two light chains
  • antigen-binding sites
  • variable regions
  • constant region
  • hinge region
  • antigen-antibody complex

Section 1 of 9Antigens

An antigen is a molecule, usually a protein or glycoprotein, on the surface of a cell or virus that stimulates an immune response.

Self
Antigens identify cells as self or non-self.
Pathogens, cells from other individuals (for example transplanted tissue or incompatible blood), toxins and cancer cells with abnormal proteins all carry non-self antigens.
Normally the immune system does not react to the body's own antigens, because T and B cells that would respond to them are eliminated or suppressed in early development.
Failure of this tolerance leads to autoimmune disease.

Section 2 of 9Antibody structure

An antibody (immunoglobulin) is a protein made by plasma cells that binds specifically to an antigen. Its structure is as follows.

Antibody structure
Two heavy chains and two light chains
Four polypeptide chains: two heavy chains and two light chains, held together by disulfide bridges, forming a Y shape.
Antigen-binding sites
Two identical antigen-binding sites at the ends of the arms, formed by the variable regions. Their amino acid sequence, and so their three-dimensional shape, differs between antibodies and is complementary to a specific antigen.
Constant region
The constant region forms the stem and is the same within a class of antibodies. It binds to receptors on phagocytes.
Hinge region
A flexible hinge region that lets the arms move to bind antigens.

Because the binding site is specific, an antibody will bind to only one antigen shape, forming an antigen-antibody complex.

Section 3 of 9How antibodies work

Agglutination
Agglutination antibodies have two binding sites so they link pathogens together in clumps, which are easier for phagocytes to engulf.
Opsonisation
Opsonisation antibodies coat the pathogen; the constant region is recognised by phagocytes, which then engulf it.
Neutralisation
Neutralisation antibodies bind toxins or the attachment proteins of viruses, preventing them from binding to host cells.
Complement activation
Complement activation leading to lysis of cells.

Section 4 of 9Primary and secondary responses

The first time a particular antigen is encountered, B cells with a matching receptor are selected and activated (clonal selection, see later lessons).

Primary response
It takes several days to produce enough plasma cells, so antibody concentration rises slowly and the person may feel ill: the primary response.
Memory cells
Some activated B and T cells become memory cells that persist for years.
On a second exposure to the same antigen, memory cells divide rapidly and differentiate into plasma cells.
Secondary response
Antibodies appear sooner, in much greater quantity, and remain longer: the secondary response.
Usually the pathogen is destroyed before symptoms develop.

Section 5 of 9Vaccination and types of immunity

A vaccine contains antigens from a pathogen, in the form of a weakened or killed organism, a purified antigen or mRNA coding for an antigen.

Vaccination and types of immunity
Active immunity
It triggers a primary response without causing disease and creates memory cells, giving active immunity. Passive immunity involves the transfer of antibodies, for example across the placenta or in breast milk, or by injection; it is immediate but temporary since no memory cells form.
Herd immunity develops when a high proportion of the population is vaccinated so the pathogen cannot spread easily, protecting those who cannot be vaccinated.
TypeMemory cells?Duration
Natural active (infection)YesLong
Artificial active (vaccine)YesLong
Natural passive (placenta, milk)NoShort
Artificial passive (antibody injection)NoShort

Section 6 of 9Monoclonal antibodies and ELISA

Antibodies can be used as tools.

In a pregnancy test, a monoclonal antibody that binds the hormone hCG is attached to a coloured marker, and a second antibody is fixed on the test strip; if hCG is present the hormone links the two so a coloured line appears.
ELISA
In an ELISA (enzyme-linked immunosorbent assay), antigens from a sample are captured by antibodies fixed in a well, a second antibody linked to an enzyme is added, and a substrate that changes colour reveals how much antigen is present.
The test is sensitive and specific, and it is used to diagnose HIV infection and allergies.

In exam questions, make clear that the specificity comes from the complementary shape of the binding site, that washing steps remove unbound antibody, and that the colour change is proportional to the amount of antigen. Controls using known negative and positive samples are required to show that the test is working.

Section 7 of 9Exam-style questions

  1. Describe the structure of an antibody and explain how its structure allows it to bind to a specific antigen. [5]5 marks

  2. Explain why the secondary immune response is faster and greater than the primary response. [4]4 marks

  3. Explain why a vaccine against a virus does not cause the disease but gives long-term protection. [3]3 marks

Section 8 of 9Key terms

TermMeaning
AntigenA molecule that stimulates an immune response.
AntibodyA protein made by plasma cells that binds a specific antigen.
Variable regionThe part of the antibody that forms the antigen-binding site.
AgglutinationClumping of pathogens by antibodies.
Memory cellA long-lived lymphocyte that gives a fast response to a repeat infection.
Active immunityImmunity from making your own antibodies.
Passive immunityImmunity from antibodies obtained from another source.
Herd immunityProtection of a population when most individuals are immune.

Section 9 of 9Exam technique

1
Saying that antibodies are cells. They are proteins; B cells and plasma cells are cells.
2
Writing that vaccines contain antibodies. They contain antigens (active immunity).
3
Mixing up the variable region (binding site) and constant region.
4
Claiming that the antibody changes shape to fit the antigen. It has a fixed complementary shape, though the hinge gives some movement.

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