immunology

Question 1

In the early 1980s, before DNA analysis had been developed, scientists investigated the genetic variation of cheetahs living in captivity. They used skin grafts to do this. They carried out skin grafts on anaesthetised animals by

• removing a small piece of skin from one animal. This animal was the recipient.
• replacing the removed skin by a piece of skin taken from another animal. This animal was the donor.
• attaching the new piece of skin with stitches.

A graft may be accepted by the recipient. It will be rejected if the recipient’s immune system recognises the antigens on the skin as foreign.

Scientists carried out skin grafts between cheetahs living in captivity and domestic cats.
The table shows the data that they obtained.

The scientists also grafted skin from one area to another on the same animal. These grafts were not rejected.

(a) (i) The scientists grafted skin from a domestic cat to a cheetah. Suggest why.

They also grafted skin from one area to another on the same animal. Explain why.

Antigens are proteins. Explain why a knowledge of antigens can show that animals are genetically similar.

Answer 1

To show whether immune response occured / because cats are (genetically) related to cheetahs;

To show that rejection did not normally occur / skin could (successfully) be grafted;

Protein / antigen production determined by alleles / genes / base sequence on DNA;
The more similar the proteins the more similar their alleles / genes / base sequence on DNA / the more they are genetically similar;

Question 1

Three injections of the vaccine are given (lines 5 to 6). Use your knowledge of immunity to suggest why.

Answer 1

More antigen;

More memory cells;

So more antibodies produced / antibodies produced quicker (if infected);

Question 2

The vaccine is made from HPV types 16 and 18 (line 5). Explain why this vaccine may not protect against other types of this virus.

____________________________________

Answer 2

Other (HPV) types have different antigens;

No memory cells for other types / memory cells not activated / antibodies cannot attach to antigen / correct antibodies not produced / antibodies are not complementary

Question 3

Most cases of cervical cancer are caused by infection with Human Papilloma Virus

(HPV). This virus can be spread by sexual contact. There are many types of HPV,

each identified by a number. Most of these types are harmless but types 16 and 18

are most likely to cause cervical cancer.

5 A vaccine made from HPV types 16 and 18 is offered to girls aged 12 to 13. Three

injections of the vaccine are given over six months. In clinical trials, the vaccine has

proved very effective in protecting against HPV types 16 and 18. However, it will be

many years before it can be shown that this vaccination programme has reduced

cases of cervical cancer. Until then, smear tests will continue to be offered to

10 women, even if they have been vaccinated. A smear test allows abnormal cells in

the cervix to be identified so that they can be removed before cervical cancer

develops.

The Department of Health has estimated that 80% of girls aged 12 to 13 need to be

vaccinated to achieve herd immunity to HPV types 16 and 18. Herd immunity is

15 where enough people have been vaccinated to reduce significantly the spread of

HPV through the population.

It will be many years before it can be shown that this vaccination programme has reduced cases of cervical cancer (lines 7 to 9). Suggest two reasons why.

Answer 3

Cancer takes years to develop / develops later in life;

Takes time for females to become sexually active / females must become sexually active to obtain data;

Few people / only teenagers vaccinated;

Question 4

Suggest one reason why vaccinating a large number of people would reduce significantly the spread of HPV through the population

Answer 4

Virus cannot replicate / is destroyed / is not carried (in vaccinated people);

Non-vaccinated people more likely to contact vaccinated people;

Question 5

Give two ways in which a pathogen may cause disease when it has entered the body

Answer 5

Produces toxins;

Neutral: infects / colonises / invades cells

Damages cells / tissues / example given e.g. cell lysis;

Question 6

Some new tests use monoclonal antibodies (lines 6–7).

(i) Explain why these antibodies are referred to as monoclonal.

Answer 6

(Antibodies) produced from a single clone of B cells / plasma cells;

Accept: hybridoma cell line instead of B cell / plasma cell
Reject: idea that antibodies are cloned

OR

(Antibodies) produced from the same B cell / plasma cell;

Question 7

The tests using monoclonal antibodies allow vets to identify brucellosis while they are still on a farm. Explain the advantages of this.

Answer 7

(d) (Rapid) treatment of carriers / infected cattle / disease;

Neutral: reference to rapid identification of infected cattle

Can isolate / cull carriers / infected cattle / infected (dairy) products not sold / consumed / tracked;

Reduces spread of disease / no need to kill / prevents the death of non-infected animals;

Question 8

Tests using monoclonal antibodies are specific (line 7). Use your knowledge of protein structure to explain why.

Answer 8

(Specific) primary structure / order of amino acids;

(Specific) tertiary / 3D structure;

(So) Only binds to / fits / complementary to one antigen;

Question 9

A disulfide bridge is labelled in the diagram above.

What is the role of the disulfide bridge in forming the quaternary structure of an antibody?

Answer 9

Joins two (different) polypeptides;

Accept holds/attaches

Accept ‘prevents polypeptide chains separating’

Question 10

Give two types of cell, other than pathogens, that can stimulate an immune response.

Answer 10

(Cells from) other organisms/transplants;

2. Abnormal/cancer/tumour (cells);
3. (Cells) infected by virus;

Question 11

Not effective in treating AIDS because

1. Number of T cells < 200 at 4 months;

Max 4 if not one of 9. or 10.

Accept 3.5 - 5 months

Reject day/week only once

2. (So) drug is not effective

OR

AIDS symptoms occur;

3. Does not remove (all) HIV (particles)

OR

Number of HIV (fairly) constant/stable

OR

(Slight) increase in HIV (over 16 months);

4. No stats test;
5. Only shows (results over) 16 months;
6. Only one person;
7. Unknown side effects (of drug);
8. No control group;

Effective in treating AIDS because

9. Number of T cells > 200 after 5 months

OR

Number of T cells increasing after 4 months;

Reject day/week only once

Accept any month after 5 months OR ‘in the long term’

10. So drug is effective

OR

AIDS symptoms relieved/removed;