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](https://www.youtube.com/watch?v=Vce3i82PJNA) **B Lymphocytes & humoral immunity** [link](https://www.youtube.com/watch?v=mAY_DLYBaIg) **T Lymphocytes & cell-medicated immunity** [link](https://www.youtube.com/watch?v=DtUmpd7mrZc) **Non-specific defences against pathogens** [link](https://www.youtube.com/watch?v=tvGiGuRle8k) **Antigens & antibodies** [link](https://www.youtube.com/watch?v=cYRe5ftY8pI) **Neutrophils, macrophages & phagocytosis** [link](https://www.youtube.com/watch?v=M3LBPQ_Mma0)

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](https://pmt.physicsandmathstutor.com/download/Biology/A-level/Notes/AQA/2-Cells/Mind-Maps/2.4%20Cell%20Recognition%20and%20the%20Immune%20System.pdf) **Question paper** [link](https://pmt.physicsandmathstutor.com/download/Biology/A-level/Topic-Qs/AQA/2-Cells/Set-B/2.4%20Cell%20Recognition%20&%20Immune%20System%20QP.pdf) **Mark scheme** [link](https://pmt.physicsandmathstutor.com/download/Biology/A-level/Topic-Qs/AQA/2-Cells/Set-B/2.4%20Cell%20Recognition%20&%20Immune%20System%20MS.pdf)

Question 35

**_SCHOOL RESOURCES_**

Answer 35

**Kerboodle link to topic** [link](https://www.kerboodle.com/api/courses/18938/interactives/111860.html) **Quizlet** [link](https://quizlet.com/179291672/cell-recognition-and-the-immune-system-flash-cards/?funnelUUID=6067cb39-5096-4152-8d2f-c1a653082a74)