Antibodies & Vaccination — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

Section 1 of 19Antibody structure

Antibody structure

Chains

• Chains four polypeptide chains held by disulfide bonds give a quaternary structure and a Y shape.
  • (two heavy and two light)

Glycoprotein

• Glycoprotein carbohydrate attached to the heavy chains makes antibodies glycoproteins.

Binding sites

• Binding sites each antibody has two identical antigen-binding sites, so it can bind two antigens.

Variable region

• Variable region it has a unique 3-D shape complementary to one antigen.

Constant region

• Constant region it binds phagocytes.

Hinge

• Hinge it gives flexibility.

Epitope

• Epitope the specific region of an antigen to which the binding site attaches.
• One large antigen may carry several epitopes, each recognised by a different antibody.

Section 2 of 19Antibody actions

Agglutination

• Agglutination antibodies cross-link pathogens into clumps that are easier to engulf and cannot spread.

Neutralisation

• Neutralisation antitoxins bind toxins, and other antibodies block binding sites so viruses cannot enter host cells.

Opsonisation

• Opsonisation antibodies coat pathogens so phagocytes recognise them.
• Phagocytes have receptors for the constant region.

Complement

• Complement antibodies trigger lysis through complement.

Immobilisation

• Immobilisation antibodies bind bacterial flagella, slowing the bacteria so phagocytes catch them more easily.

Section 3 of 19Types of immunity

Types of immunity

Active immunity

• Active immunity it involves clonal selection and expansion and makes memory cells.

Passive immunity

• Passive immunity it gives immediate but temporary protection because the antibodies are broken down.

Natural versus artificial

• Natural versus artificial vaccination gives artificial active immunity.

Section 4 of 19Vaccines

Content

• Content a vaccine contains antigens but cannot cause disease; boosters increase memory cell numbers.

Live attenuated vaccines

• Live attenuated vaccines weakened whole pathogens that multiply only slowly without causing disease.
  • (for example MMR)
• They give a strong, long-lasting response, often after one dose.
• They are unsafe for people with weakened immune systems.

Inactivated vaccines

• Inactivated vaccines killed whole pathogens or parts such as proteins or toxoids (for example diphtheria).
  • (for example the polio injection)
• They cannot cause disease.
• The response is weaker, so boosters and sometimes adjuvants are needed.

Section 5 of 19How vaccination gives immunity

1

A vaccine containing antigen is injected.

2

Antigen-presenting cells present the antigen to specific helper T-cells and B-cells.

3

Clonal selection and expansion occur (primary response).

4

Plasma cells make antibodies and memory cells form.

5

On later infection memory cells divide rapidly.

6

Large quantities of antibody destroy the pathogen before symptoms (secondary response).

Section 6 of 19Vaccination programmes

Best targets

• Best targets diseases with a single stable antigen and no animal reservoir, such as smallpox.
  • (ring vaccination)

Smallpox eradication

• Smallpox eradication the virus was stable, the vaccine was live and easy to store, and symptoms were easy to identify.

Herd immunity threshold ≈ 1 − 1/R₀; for measles R₀ ≈ 15, so about 93 to 95 per cent coverage is needed.

Measles

• Measles it still occurs because the vaccine needs two doses and very high coverage, and some parents refuse vaccination.

Section 7 of 19Why some vaccines are hard

1Antigenic variation

• Antigenic variation influenza and HIV change their antigens.

2Hidden pathogens

• Hidden pathogens those inside cells, infecting immune cells or in the gut lumen (cholera) escape antibodies.
  • (HIV in helper T-cells)

3Other problems

• Other problems many strains, poor response in malnourished people, cold-chain storage and public refusal.

4Cholera, malaria and influenza

• Cholera, malaria and influenza vaccines are hard to produce for these reasons.

Section 8 of 19Making monoclonal antibodies

1

Inject a mouse with the antigen so its B-cells make plasma cells.

2

Extract plasma cells from the mouse spleen.

3

Fuse them with myeloma cells to form hybridomas (using polyethylene glycol or an electric pulse).

4

Culture hybridomas in a selective medium where unfused cells die.

5

Screen the surviving clones to find the one making the required antibody.

6

Grow that clone in large quantities.

7

Collect and purify the monoclonal antibodies.

Section 9 of 19Uses of monoclonal antibodies

Reason for fusion

• Reason for fusion plasma cells cannot divide, so fusing them with myeloma cells gives hybridomas that both divide and secrete one specific antibody.

Diagnosis

• Diagnosis pregnancy tests, detecting HIV and locating tumours.

Treatment

• Treatment for example trastuzumab (Herceptin) for HER2-positive breast cancer, and delivering drugs to cancer cells.

Humanised antibodies

• Humanised antibodies mouse antibodies are themselves foreign antigens, so they are humanised to reduce the patient's immune response.

Section 10 of 19Primary and secondary responses

Primary response

• Primary response on first exposure to an antigen, antibodies appear slowly and in small amounts.

Secondary response

• Secondary response on second exposure it is much faster and larger because memory cells are already present.

Graphs

• Graphs antibody concentration against time shows a bigger, faster peak for the second exposure.

Section 11 of 19Active immunity

Meaning

• Meaning the person's own immune system makes antibodies.

Natural

• Natural through infection.

Artificial

• Artificial through vaccination.

Memory cells

• Memory cells they are formed, so immunity is long-lasting.

Section 12 of 19Passive immunity

Meaning

• Meaning it is acquired without an immune response, as the antibodies are not made by the person.

Memory cells

• Memory cells none are formed, so there is no secondary response and protection is temporary.

Natural examples

• Natural examples the foetus gets antibodies across the placenta, and babies get colostrum carrying IgA.

Artificial example

• Artificial example injected antibodies.

Section 13 of 19Active and passive immunity compared

Feature	Active	Passive
Production of antibodies	Produced by the body	Not produced by the body
Memory cells	Formed	Not formed
Speed	Slow to take effect	Immediate
Duration	Long-term	Short-term

Section 14 of 19Vaccines

Definition

• Definition a suspension of antigens intentionally put into the body to induce artificial active immunity.

Response

• Response antibodies are released by plasma cells and memory cells are formed.

Route

• Route by injection or orally.

Live attenuated vaccines

• Live attenuated vaccines whole weakened bacteria or viruses that multiply slowly so the body recognises the antigens and mounts a primary response.
• They tend to give a stronger and longer-lasting response.

Inactivated vaccines

• Inactivated vaccines whole killed pathogens, or small parts (subunit) such as proteins or sugars, or harmless toxins called toxoids.

Section 15 of 19Vaccination and herd immunity

Safe vaccines

• Safe vaccines vaccines against many pathogens have pushed a number of childhood diseases to the verge of extinction.
• Vaccines against mumps, chickenpox and whooping cough are given as part of an immunisation schedule.

Herd immunity

• Herd immunity when enough of a population is vaccinated the pathogen cannot spread easily.
• Unvaccinated people such as babies and the immunocompromised are indirectly protected because few infected people can pass it on.

Section 16 of 19More from the specification

More from the specification

Monoclonal Antibody Production

• The hybridoma method involves fusing antibody-producing B-lymphocytes with immortal myeloma cells to produce clones making identical, specific monoclonal antibodies.

Antibody Structure and Binding Sites

• An antibody molecule contains variable regions forming specific antigen-binding sites that fit complementary epitopes on the surface of foreign antigens.

Diagnostic and Cancer Therapies

• Monoclonal antibodies can target specific antigens on cancer cell surfaces, delivering radioactive payloads or triggering immune destruction in blood tissues.

Constant and Variable Regions

• The constant region of an antibody dictates its interaction with immune effector cells and phagocytes,
• while the variable region ensures high specificity.

Variable region

• The arm tip that forms the antigen-binding site.

Receptor protein

• Protein on the postsynaptic membrane that binds neurotransmitter.

Specific

• Able to act on only one type of substrate.

Hybridoma

• A fused plasma cell and tumour cell that makes one antibody.

Cancer

• Disease in which cells divide uncontrollably and spread.

Tumour

• A lump of cells formed by uncontrolled division.

Monoclonal antibody

• Identical antibodies from one clone, all binding the same epitope.

Section 17 of 19More on this topic

• Herd immunity
  • when a large proportion is vaccinated, spread becomes difficult and even the unvaccinated, such as those who cannot be vaccinated, are protected.

Section 18 of 19Key terms

Term	Meaning
Antibody	A Y-shaped glycoprotein with specific variable regions that binds an antigen.
Epitope	The region of an antigen that an antibody binds.
Hybridoma	A fusion of a plasma cell and a myeloma cell that divides and secretes one antibody.
Herd immunity	Protection of a population when most individuals are immune.
Attenuated vaccine	A vaccine of weakened live pathogens.
Adjuvant	A substance that boosts the response to a vaccine.

Term	Meaning
Primary response	The slow response on first exposure to an antigen.
Secondary response	The faster, larger response on later exposure.
Colostrum	The first breast milk, delivering IgA.
Subunit vaccine	A vaccine of small parts of a pathogen.
Toxoid	A harmless form of a toxin.
Immunisation schedule	A planned timetable of childhood vaccines.

Section 19 of 19Exam technique

1

Saying antibodies are made by T cells — plasma cells (B cells) make them.

2

Saying passive immunity gives memory cells — it does not and it is temporary.

3

Saying live vaccines are safe for everyone — they are unsafe for people with weak immune systems.

4

Saying herd immunity needs 100 per cent vaccination — the threshold is about 1 − 1/R_{0}.

1

Saying passive immunity is long-lasting — it is short-term.

2

Saying active immunity is immediate — it is slow to take effect.

3

Saying herd immunity needs no one to be vaccinated — it needs a large proportion immune.

4

Saying inactivated vaccines can cause disease — they contain no living pathogen.