3. 2. 4 Cell Recognition and the Immune System - NOT DONE YET AT SCHOOL

Question 1

Why do cells have specific molecules on its surface?

Answer 1

Each type of cell has specific molecules on its surface to identify it. These molecules include proteins & enable the immune system to identify:

• Pathogens
• Cells from other organisms of the same species
• Abnormal body cells
• Toxins

Question 2

What is the definition of antigen?

Answer 2

Antigens are molecules present on the surface of cells, which trigger an immune response. Antigens include proteins on cell surface membranes. The immune system detects antigens in order to identify a cell. Antigens are typically unique and specific to particular cells and are also specific to particular pathogens.

Question 3

What is antigen variability?

Answer 3

Some pathogens exhibit antigen variability

The antigens present on their surface change frequently due to genetic mutations

Question 4

What is the effect of antigen variability in terms of disease and disease prevention?

Answer 4

Antigen variability poses a problem for many mammal hosts as lymphocytes & memory cells produce a specific immune response

• The surface receptors on lymphocytes & memory cells are complementary in shape to only one antigen
• When the antigen on a pathogen changes the lymphocytes & memory cells can no longer bind
• As a result there is no secondary immune response
• The host gets infected & suffers from the disease again

Question 5

Name two types of viruses that exhibit antigen variability?

Answer 5

The cold and flu virus are common pathogens that exhibit antigen variability

Individuals can catch the cold & flu year after year as the antigens on the virus change & are not recognised by their immune system

Question 6

What are the two different types of white blood cells?

Answer 6

• Phagocytes
• Lymphocytes

Question 7

What happens during phagocytosis?

Answer 7

1 - Pathogens are engulfed by phagocytes in the form of vesicles which are formed on the cell-surface membrane

2 - Chemical products of the pathogen act as attractants which draw the phagocyte towards it

3 - Phagocytes attach themselves to the surface of the phagosome

4 - They engulf the pathogen to form a vesicle known as phagosome

5 - Enzymes within the Lysosomes join with the phagosome and release their contents. The enzymes within the Lysosomes digest the pathogen

6 - The soluble products of the pathogen are absorbed into the cytoplasm of the phagocyte

Question 8

What is the response of T lymphocytes to a foreign antigen?
(the cellular response)

Answer 8

Antibodies attach to a specific antigen & make it easier for the immune cells to destroy the antigen. T lymphocytes attack antigens directly & help control the immune response. They also release chemicals known as cytokines which control the entire immune response.

Question 9

What is the role of antigen presenting cells?

Answer 9

An antigen presenting cell (APC) presents antigens to helper T cells to activate T cells during a cellular response

Question 10

What is the definition of antibody?

Answer 10

A blood protein produced in response to & counteracting a specific antigen. Antibodies combine chemically with substances which the body recognises as alien, such as bacteria, viruses & foreign substances in the blood

Question 11

Describe the structure of an antibody

Answer 11

1 - Antibodies are Y-shaped. Each antibody is a Y-shaped molecule with four polypeptide chains making it up. There are two long heavy chains & 2 short light chains.

2 - Antibodies have a constant region. Every antibody has the same constant region. The antibody uses this to bind to phagocytes.

3 - Antibodies have a variable region. The variable region has a unique structure that. is different for each and every antibody molecule. This variable region is the antigen binding site of the antibody by which the antibody molecule can recognise & bind to a particular antigen. It is like the active site of the enzyme.

4 - Antibodies have hinge regions. Antibodies have flexible hinge regions, which allow the branches of the Y to move away from each other. This makes antibodies more flexible, so it can bind to multiple antigens.

Question 12

Describe the response of B lymphocytes to a foreign antigen

Answer 12

B lymphocytes also called B cells, create a type of protein called an antibody. These antibodies bind to pathogens, foreign substances/antigens, such as toxins to neutralise them

Question 13

How does an antigen prevent infection?

Answer 13

An antigen can bind to a virus, preventing it from entering a normal cell & causing infection

Question 14

Describe clonal selection

Answer 14

When an antigen encounters the immune system, its epitopes eventually will react only with B lymphocytes with B-cell receptors on their surface that more or less fit, this activates those B-lymphocytes. This process is known as clonal selection.

Question 15

How are monoclonal antibodies produced & released?

HUMORAL RESPONSE

Answer 15

Monoclonal antibodies are produced naturally in the body during the immune response. Specifically the humoral response. Here monoclonal antibodies are produced by one type of B cells (B lymphocytes), which releases the specific antibody that is complementary to the antigen of the pathogen being responded to

Question 16

What are the different types of T cells?

Answer 16

T cells come in 3 major groups

1 - T helper cells (T_H cells)

2 - T killer cells ( T_C - for cytotoxic, a term meaning kills cells)

3 - Regulatory T cells (T_reg)

Question 17

What is the role of helper T cells (T_H cells) in stimulating cytotoxic cells (T_C cells)?

Answer 17

T helper cells (T_H) play an important role in activating & co-ordinating the rest of the immune response.

The cytokines released by helper T cells help activate cytotoxic T cells (T_CC). Cytotoxic T cells send out molecules to fight the infection. Cytotoxic T cells can also recognise infected cells & directly kill them to prevent further infection.

Question 18

What is the role of helper T cells (T_H cells) in stimulating B cells?

Answer 18

Helper T cells (T_H cells) stimulate B cells to make antibodies and help killer cells develop.

Question 19

What is the difference between T cells & B cells?

Answer 19

T cells use cytokines as messenger molecules to send chemical instructions to the rest of the immune system

B cells produce antibodies to fight infection

Question 20

What is the role of helper T cells (T_H cells) in stimulating phagocytes?

Answer 20

As well as stimulating cytotoxic T cells, helper T cells also stimulate phagocytes. Phagocytes engulf & destroy infected cells.

Question 21

Describe the formation of an antigen-antibody complex

Answer 21

• An antigen & it’s complementary antibody have complementary molecular shapes
• This means their molecular structures fit into each other
• When an antibody collides (randomly) with a foreign cell that possesses non-self antigens with a complementary shape, it binds with one of the antigens
• When this occurs, the two molecules combine to form an antigen-antibody complex

Question 22

What is agglutination?

Answer 22

• Antibodies have at least two antigen-binding sites
• This means they can bind to more than one bacterium or virus at the same time
• This causes groups of the same pathogens to become clumped together
• This is known as agglutination

MEMORY AID: THINK AG -GLU- TINATION THEY GLUE TOGETHER

Question 23

ANTIGEN-ANTIBODY COMPLEX

Describe how agglutination & phagocytosis lead to the destruction of antigens

Answer 23

• The binding of antibodies to the antigens either neutralises the pathogen or acts like a marker to attract phagocytes to engulf & destroy the pathogens.
• Due to agglutination, phagocytes can often phagocytose many pathogens at the same time as they are all clumped/glued together

Question 24

What is a primary immune response?

Answer 24

A primary immune response responds to a newly encountered antigen

Question 25

What is a secondary immune response?

Answer 25

A secondary immune response responds to a previously encountered antigen

Question 26

Describe the role of plasma cells & memory cells in producing a primary immune response

Answer 26

• After clonal selection & expansion, the B-lymphocytes that have become plasma cells secrete lots of anti-body molecules (specific to the antigen) into the blood, lymph or linings of the lungs or guts.
• These plasma cells are short lived (their numbers drop off after several weeks) but the antibodies they have secreted stay in the blood for a long time
• The other B-lymphocytes become memory cells that remain circulating in the blood for a long time
• The primary response to newly encountered pathogens is relatively slow.

Question 27

Describe the role of plasma cells & memory cells in producing a secondary immune response

Answer 27

• If the same antigen is found in the body a second time, the memory cells recognise the antigen divide very quickly and differentiate into plasma cells (to produce antibodies) & more memory cells
• This response is very quick, meaning the infection can be destroyed & removed before the pathogen population increases too much & symptoms of the disease develops
• This secondary response to a previously encountered pathogen is relative to the primary immune response, extremely fast

Question 28

Explain the use of vaccines to provide direct protection for individuals & populations against disease

Answer 28

• Vaccination is the introduction into the body of a vaccine containing disease antigens by injection or mouth in order to induce artificial immunity
• Vaccines work by injecting weakened/dead pathogens into the body to stimulate a immune response, (primary response) to form memory cells against the specific antigen, which then stimulates a secondary response to infection, which destroys the pathogens quickly, as such creating protection for individuals
• When a significant proportion of the population is vaccinated this creates protection for individuals who have not yet been vaccinated & the population (herd immunity)

Question 29

What is the concept of herd immunity?

Answer 29

Herd immunity arises when a sufficiently large proportion of the population is vaccinated (and therefore immune) which makes it difficult for a pathogen to spread within that population. Those who are not immunised are protected and are unlikely to contract it as the levels of the disease are so low.

Question 30

What is active immunity?

Answer 30

Active immunity results when exposure to a disease organism triggers the immune system to produce its own antibodies to that disease

Active immunity can be acquired through natural immunity or artificial immunity

An example of active natural immunity is when you become immune after catching a disease

An example of active artificial immunity is when you have been given a vaccine containing a harmless dose of antigen

Question 31

What is passive immunity?

Answer 31

Passive immunity is immunity that is acquired through the transfer of antibodies from one individual to another, your immune system doesn’t produce any antibodies of its own

Passive immunity can be acquired through natural immunity or artificial immunity

An example of passive natural immunity is when a baby becomes immune, either through pregnancy or though the mothers breast milk

An example of passive artificial immunity is when you become immune after being injected by antibodies from someone else usually collected through blood doners

Question 32

What is the difference between active & passive immunity?

Answer 32

Active immunity
- Requires exposure to antigen
- Protection takes a while to develop
- Memory cells are produced
- Protection is long term because the antibody is produced (after activation of memory cells) due to the complementary antigen already being present

Passive immunity
- Does not require exposure to antigen
- Protection is immediate
- Memory cells aren’t produced
- Protection is short term because the antibodies given are broken down

Question 33

YOUTUBE LINKS TO SUBTOPIC

Answer 33

Cell recognition & the immune system
link

B Lymphocytes & humoral immunity
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T Lymphocytes & cell-medicated immunity
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Non-specific defences against pathogens
link

Antigens & antibodies
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Neutrophils, macrophages & phagocytosis
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Question 34

PHYSICS & MATHS TUTOR LINKS

Answer 34

PMT Cell recognition & the immune system flashcards
[link](https://www.physicsandmathstutor.com/biology-revision/a-level-aqa/cells/cell-recognition-and-the-immune-system-flashcards/0

PMT Cell recognition & the immune system mindmap
link

Question paper
link

Mark scheme
link

Question 35

SCHOOL RESOURCES

Answer 35

Kerboodle link to topic
link

Quizlet
link