Cell Recognition + the Immune System

Question 1

What are the defence mechanisms our bodies have against pathogens?

Answer 1

• preventing entry of pathogens by physical + chemical defences (e.g. skin, mucous membranes, tears + saliva)
• inflammation of region invaded by pathogen (non-specific inflammatory response)
• recognising ‘foreign’ cells + targeting any pathogenic cells (specific immune response)

Question 2

How do are bodies recognise ‘foreign’ cells?

Answer 2

• cells have specific molecules (usually proteins) on their surface, enabling them to be identified
  
  • proteins are often part of phospholipid bilayer (e.g. glycoproteins)
  • but glycolipids also act as similar markers
• allows body to recognise its own cells + foreign cells

Question 3

What do these surface proteins enable the body to recognise?

Answer 3

• pathogens (e.g. bacteria, fungi, viruses)
• abnormal body cells (e.g. cancerous cells)
• toxins (e.g. some pathogens, like cholera, release chemicals into the blood)
• cells from other organisms of the same species (e.g. organ transplants)

Question 4

What are the surface molecules use by the body to identify cells, vital particles + toxins known as?

Answer 4

• antigens

Question 5

What is the definition of antigen?

Answer 5

• molecules, present on surface of cells, that generate an immune response by lymphocyte cells when detected in the body

Question 6

What is an example of the importance of antigens defending against pathogens?

Answer 6

• white blood cells (phagocytes) have surface proteins that act as receptors + bind to antigens on surface of pathogens
• enables pathogens to be engulfed + digested
• antigens found on pathogen can be presented on surface of phagocyte
• recruits other cells of immune system: leads to specific immune response

Question 7

What does every cell in the human body have to identify it?

Answer 7

• markers

Question 8

What do antigens allow?

Answer 8

• cell-to-cell recognition

Question 9

Where are antigens found?

Answer 9

• cell surface membranes
• bacterial cell walls
• surfaces of viruses

Question 10

What are self antigens?

Answer 10

• antigens produced by organism’s own body cells
• doesn’t stimulate an immune response

Question 11

What are non-self antigens?

Answer 11

• antigens not produced by organism’s own body cells
• stimulates an immune response

Question 12

What is antigen variability?

Answer 12

• antigens present on surface of some pathogens change frequently due to genetic mutations

Question 13

Why does antigen variability pose a problem for the immune system of many mammal hosts?

Answer 13

• bc lymphocytes + memory cells produce a specific immune response as they have surface receptors complementary to only 1 antigen
• meaning when antigen on pathogen changes, lymphocytes + memory cells can no longer bind = no secondary immune response
• host gets infected + suffers from disease again

Question 14

What are common pathogens that exhibit antigen variability?

Answer 14

• the cold virus
• flu virus

Question 15

What are the diff. types of pathogen?

Answer 15

• viruses
• bacteria
• fungi
• protozoans

Question 16

What are phagocytes?

Answer 16

• white blood cells produced continuously in the bone marrow
• stored in bone marrow before being distributed around body in blood

Question 17

What is the function of phagocytes?

Answer 17

• responsible for removing dead cells + invasive microorganisms
• carry out the non-specific immune response

Question 18

What are the 2 main types of phagocyte?

Answer 18

• neutrophils
• macrophages

Question 19

What is phagocytosis?

Answer 19

• process of recognising + engulfing a pathogen

Question 20

What is the difference between neutrophils (n) + macrophages (m)?

Answer 20

• (m) larger than (n)
• (m) are long-lived cells whilst (n) are short-lived cells
• (n) remain in blood whereas (m) move into organs

Question 21

How do phagocytes carry out phagocytosis?

Answer 21

• phagocytes move towards pathogen via Chemotaxis (attracted by chemicals released by pathogen)
• receptors on phagocyte attach to antigens on pathogen
• phagocyte engulfs pathogen via endocytosis forming a phagosome
• a lysosome bind to phagosome + releases lysozymes that hydrolyse + digest the pathogen
• phagocyte absorbs products + displays antigens on cell membrane (antigen presenting cell)

Question 22

What are the 2 types of white blood cell?

Answer 22

• phagocytes
• lymphocytes

Question 23

What are lymphocytes?

Answer 23

• white blood cells involved in specific immune response
• smaller than phagocytes
• have large nucleus filling most of cell
• produces in bone marrow before birth

Question 24

What are the 2 types of lymphocytes?

Answer 24

• T-lymphocytes (T cells)
• B-lymphocytes (B cells)

Question 25

Where do immature T-lymphocytes mature?

Answer 25

• leave bone marrow to mature in thymus

Question 26

How are T-lymphocytes involved in the cell mediated (cellular immune) response?

Answer 26

• once a pathogen has been engulfed + destroyed by a phagocyte, the antigens are presented on the cell surface (antigen presenting cell)
• mature T-lymphocytes have T cell receptors on their surface specific to 1 antigen that can attach to complementary antigens on the APC
• activating T-lymphocytes to divide by mitosis, to replicate + make a large number of clones that differentiate into 2 main types of T cell

Question 27

What are the 2 main types of T cell that T-lymphocytes differentiate into?

Answer 27

• helper T cells
• cytotoxic T cells (aka killer T cells)

Question 28

When will T-lymphocytes bind to an antigen?

Answer 28

• if it’s present on the surface of an antigen-presenting cell

Question 29

What is an antigen-presenting cell?

Answer 29

• a host cell that’s been invaded by a pathogen + is displaying the antigen on its cell surface membrane

Question 30

What does an antigen-presenting cell do?

Answer 30

• present antigens from toxins, foreign cells + ingested pathogens
• helps recruit cells of immune system to produce a specific immune response
• once T cell receptor binds to complementary antigen on APC, it becomes sensitised + starts dividing to produce clones

Question 31

What are e.g.s of cells that can present antigens?

Answer 31

• common = macrophages + dendritic cells
• less common = neutrophils

Question 32

What is the role of helper T cells?

Answer 32

• assist other white blood cells in immune response
• release cytokines (hormone-like signals) which stimulate:
  - maturation of B-lymphocytes into antibody-secreting plasma cells
  - production of memory B cells
  - activation of cytotoxic T cells: destroys virus infected cells + tumour cells
  - an inc. rate of phagocytosis

Question 33

What is the role of cytotoxic T cells (T killer cells)?

Answer 33

• patrol body in search of antigen-presenting body cells
• attach to foreign antigens on cell surface membranes of infected cells + secrete toxic substances that kill infected body cells, along w pathogen inside
  - perforins secreted by cytotoxic T cells pierce cell surface membrane, allowing toxins to enter

Question 34

Where are B-lymphocytes (B cells) found?

Answer 34

• remain in bone marrow until mature + then spreads through body, concentrating in lymph nodes + spleen

Question 35

How are B-lymphocytes (B cells) involved in the humoral immune response?

Answer 35

• every B lymphocyte has a specific antibody on their cell surface membrane (B cell receptors) that’s complementary to 1 antigen
• when a complementary B cell receptor binds to an antigen, the B cell takes in antigen by endocytosis + displays it on its cell surface membrane (APC)
• the antigen presenting B cell then binds to a complementary helper T cell receptor, activating B cell to rapidly divide by mitosis [clonal expansion]
• clones then differentiate into plasma cells or memory B cells (primary response)
  
  • most are plasma cells: produce monoclonal antibodies
  • memory B cells: long-lived, + divides rapidly into plasma cells if reinfected w same pathogen = produces lots of antibodies rapidly so pathogen destroyed before symptoms occur (secondary response)

Question 36

What is the definition of an antibody?

Answer 36

• proteins produced by plasma cells, as a part of the immune response, which binds to antigens

Question 37

What is the structure of an antibody?

Answer 37

• quaternary structure protein (4 polypeptide chains)
• 2 heavy chains (long) bonded to 2 light chains (short) by disulphide bonds
• has a constant region
• has a variable region (antigen binding site = specific + complementary)

Question 38

What is agglutination?

Answer 38

• a complementary antibody binds to an antigen on a pathogen (antigen-antibody complex)
• antibodies: slightly flexible = can bind to 2 antigens + clump them together
• either neutralises pathogen or makes it easier for phagocytes to locate + destroy

Question 39

What do B cells differentiate into?

Answer 39

• plasma cells
• memory B cells