3.3.4 The Human Immune System · 3. Interactions with the Environment | Aqa GCSE Combined Science Synergy 8465: Life And Environmental Sciences

Section 1 of 9Antigens and antibodies

Every cell has molecules on its surface that identify it. Pathogens carry foreign molecules called antigens, usually proteins, on their surface. The body's own cells have antigens that the immune system recognises as 'self' and does not attack. When foreign antigens are detected, the body mounts a specific immune response.

Antibodies are proteins made by white blood cells called B lymphocytes. The antibody has a variable region whose three-dimensional shape is complementary to one particular antigen; an antibody against measles will not fit the antigens of flu. Because antibodies are proteins, their shape depends on the order of amino acids, as in any protein, and they can be denatured.

Section 2 of 9The primary response

1

A pathogen enters the body.

Its antigens are detected by a lymphocyte that has a matching antibody on its surface, sometimes with the help of a phagocyte that displays pieces of the pathogen.

2

The lymphocyte is activated and divides rapidly by mitosis, producing a clone of identical cells. This takes several days.

3

The clone cells release large amounts of the specific antibody into the blood.

4

Antibodies bind to the antigens, causing pathogens to clump together, blocking their entry into cells or marking them so that phagocytes engulf them.

Some antibodies cause bacteria to burst.

5

The infection is cleared and the symptoms disappear. Antibody levels fall again.

Some lymphocytes become memory cells, which stay in the blood and lymph for many years. Meanwhile some types of lymphocyte produce antitoxins to neutralise toxins and other cells (T cells) destroy infected body cells directly, but at GCSE the focus is on antibodies and memory cells.

Section 3 of 9The secondary response and immunity

If the same pathogen enters the body again, the memory cells recognise the antigen straight away.

They divide very quickly and produce antibodies much faster and in far greater quantity than the first time.

Secondary response

This is the secondary response.

Immune

The pathogen is destroyed before it can multiply enough to cause symptoms, so the person is immune.

The immunity is specific: it protects only against that pathogen.

	Primary response	Secondary response
Time before antibodies appear	Several days	A day or less
Amount of antibody	Low peak	Much higher peak
Duration	Short	Longer lasting
Symptoms	Yes	Usually none

Section 4 of 9Interpreting a graph of antibody concentration

A typical graph has time on the x-axis and antibody concentration on the y-axis.

1

After the first exposure, the line stays flat for a few days (a lag phase), then rises to a modest peak and falls.

2

After a second exposure the line rises almost immediately to a peak several times higher.

3

When describing this: state the starting point, the time delay, the height of each peak with units, and then compare.

4

For example, 'after the first infection antibody concentration reached 20 units on day 10,

but after the second it reached 120 units by day 3',
so the second response was six times higher (120 / 20) and seven days faster.

Always link the explanation to memory cells: they are already present, so clone production starts at once.

Section 5 of 9Why you can catch some diseases repeatedly

Immunity to measles lasts a lifetime but you can catch colds or flu many times.

1

This is because these viruses mutate frequently, producing new antigens.

2

Memory cells for the earlier version do not recognise the changed antigen, so a primary response is needed again.

3

This is why flu vaccines are reformulated each year.

4

Some pathogens, such as HIV, directly attack the immune cells; others hide inside cells, and some bacteria have capsules that hide their antigens.

5

Allergies and autoimmune diseases are cases where the immune system reacts to harmless substances or to the body's own cells,

which shows how immune responses can also cause harm.

Section 6 of 9Passive immunity and the role of T cells

Immunity can also be gained passively, when ready-made antibodies are passed to a person.

1

A baby receives antibodies from its mother across the placenta and in breast milk,

which protects it in the first months;
- an injection of antibodies can be given after exposure to a dangerous pathogen such as tetanus or rabies.

2

Passive immunity gives immediate protection but it is short-lived, because the person has not made memory cells.

3

Active immunity, by contrast, is slower to start but lasts much longer.

4

Besides B lymphocytes, other lymphocytes called T cells destroy body cells that have been infected by viruses, and help to coordinate the response,

so that the immune system acts as a team.

Section 7 of 9Exam-style questions

Explain why a person who has had chickenpox is usually immune to the disease for the rest of their life. [3]3 marks
A graph shows antibody concentration peaks at 15 units 10 days after the first exposure and at 90 units 3 days after the second. Calculate how many times greater the second peak is and suggest why. [3]3 marks
Explain why antibodies for one pathogen do not protect against a different one. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Antigen	A molecule that triggers an immune response.
Antibody	A protein that binds to a specific antigen.
Memory cell	A long-lived lymphocyte that gives a fast response on re-exposure.
Primary response	The first, slower response to a pathogen.
Secondary response	The faster, stronger response on second exposure.
Immunity	Protection against a pathogen through the immune system.
Clone	A group of genetically identical cells.
Lymphocyte	A white blood cell that produces antibodies.

Section 9 of 9Exam technique

1

Saying that antibodies engulf pathogens. They bind to antigens; phagocytes engulf.

2

Believing the same antibody works against every pathogen.

3

Confusing antigen (on the pathogen) with antibody (made by the body).

4

Saying memory cells make antibodies continuously. They stay ready and respond quickly on re-exposure.

5

Explaining flu re-infection without mentioning mutation of antigens.