Chapter 5: Responding to Antigens and Acquiring Immunity

Question 1

Define antigen

Answer 1

• Antigens are molecules or parts of molecules that stimulate an immune response

Question 2

Describe the difference between self and non-self antigens

Answer 2

• Self antigens are antigens on cells that are recognised by self-receptors as being part of the same body
• Non-self antigens are antigens that do not belong to the body’s own cells

Question 3

Describe the difference between MHC-I and MHC-II self markers

Answer 3

• MHC-I is a type of major histocompatibility complex found on all nucleated cells
  
  • They allow cells to be recognised as “self”
• MHC-II is a type of major histocompatibility complex found on specific white blood cells, including antigen-presenting cells, involved in the adaptive immune response

Question 4

Explain what a pathogen is

Answer 4

• Pathogens are agents that cause disease to a host

Question 5

Explain the difference between pathogens and antigens

Answer 5

• Antigens are substances such as pollen, bacteria and viruses that trigger an immune response
• Pathogens are harmful agents such as bacteria that can cause disease

Question 6

List the 6 main types of pathogen

Answer 6

• Bacteria, worms, fungi, protozoa, viruses and prions

Question 7

Compare infection and disease

Answer 7

• Infection is the invasion and growth of a pathogen in the body
• Disease is a condition that impairs the normal functioning of an organ, structure or system of an organism

Question 8

Describe the difference between cellular and non-cellular pathogens

Answer 8

• Cellular pathogens are living organisms that are able to reproduce independently
• Non-cellular pathogens are non-living and require a host to reproduce

Question 9

Compare extracellular and intracellular pathogens

Answer 9

• Extracellular pathogens are targeted by the humoral immune response
  
  • Cannot survive inside the phagocyte once ingested
  • In the body but have yet to enter cells
• Intracellular pathogens are targeted by the cell-mediated immune response
  
  • Can survive inside of host cells/phagocytes
  • Have entered cells

Question 10

Explain how viruses damage cells

Answer 10

• Viruses cause disease by killing body cells (cell lysis) and uses the cell’s organelles to reproduce
• Virus adheres to a host cell, injecting its viral DNA
• Viral nucleic acid moves to nucleus where it is transcribed
• Viral mRNA is then translated and viral protein is packaged
• The infected host cell bursts as its plasma membrane disintegrates and viral particles are released into the extracellular fluid from where they can infect other cells

NOTE: Viruses have either DNA or RNA that is surrounded by a protein shell (capsid).

Question 11

Explain how bacteria cause disease

Answer 11

• If bacteria multiplies in areas in which they are not normally found, they can cause disease

Question 12

Define allergen

Answer 12

• Allergens are substances that cause allergic response

Question 13

Outline the general process of an allergic response

Answer 13

• Allergic response is a reaction to normally harmless antigens
• Sensitization → initial exposure to allergen
  
  • Allergen enters the bloodstream, B cells differentiate into plasma cells, plasma cells produce antibodies and antibodies attach to mast cells
  • Mast cells become ‘primed’ with IgE/antibodies specific for the allergen (able to recognise and respond to the allergen)
• Allergic reaction → secondary exposure to the same allergen
  
  • Allergen binds to antibodies forming cross links on mast cells, histamine is released from the mast cells and an allergic reaction occurs
  • When an allergen binds to more than one antibody a cross link will form (this results in the release of histamine)
  • No cross link = no histamine produced

Question 14

Describe the role of mast cells, with clear reference to allergic response

Answer 14

• Mast cells are immune cells that release histamines via degranulation, which causes inflammation

Question 15

Describe 3 different physical barriers in animals

Answer 15

• Intact skin → the constant shedding of surface cells is an effective barrier against the entry of pathogens
• Ear wax → reduces the access that pathogens have to the ear drum and ear canal as well as protects the ear from dust and other foreign particles
• Nasal hair and mucus → trap and prevent the entry of pathogens

Question 16

Describe 3 different chemical barriers in animals

Answer 16

• Lysozyme → an enzyme present in sweat, tears and saliva that kills pathogens
• Sebum → an oily secretion produced by sebaceous glands that provides a protective and antimicrobial film on the skin
• Stomach acid → including digestive enzymes kill pathogens that enter the digestive tract

Question 17

Explain how microbiological barriers in animals can prevent infection

Answer 17

• Microbiological barriers include harmless bacteria that occur naturally within the body that inhibit the growth of pathogens
• They do this by secreting antimicrobial chemicals and by outcompeting them for nutrients and adhesion site

Question 18

Describe 3 different physical barriers in plants

Answer 18

• Waxy cuticle → the waxy coat on leaves that prevent infection
• Formation of galls → limits the spread of pathogens
• Thorns → protect plants from grazing animals such as cattle and sheep

Question 19

Describe chemical barriers in plants

Answer 19

• Caffeine → helps to prevent fungi and insects from invading the plant
• Antimicrobial enzymes → prevent pathogens from entering the plant
• Antimicrobial chemicals → produced by plants to disrupt bacterial cell membranes and deter predators

Question 20

Describe components of the first line of defence that work against viruses

Answer 20

• Mucous membranes make it difficult to viruses to adhere
• Nose hairs (cilia) are a physical barrier to the virus
• Acidic environment of the stomach kill viruses

Question 21

Describe the innate immune response, with specific reference to the second line of defence

Answer 21

• The innate immune response is an inborn system that lacks specifity and memory
• The second line of defence involves cell-mediated innate immunity, humoral innate immunity, as well as inflammation and fever
• The second line of defence is in effect once the first line of defence has been breached or when pathogens have entered tissue or the bloodstream

Question 22

Explain the purpose of a fever

Answer 22

• Increased body temperatures enhance the performance of immune cells, making them work more efficiently
• Heat can also kill bacteria and viruses as well as denature bacterial enzymes

NOTE: Fevers are part of the second line of defence.

Question 23

Describe the process of phagocytosis

Answer 23

• A pathogen is identified by a pattern recognition receptor and is engulfed in the plasma membrane of a phagocyte (macrophages, neutrophils or dendritic cells)
• The pathogen is engulfed in a vesicle called a phagosome
• Lysosomes fuse with the phagosome
• Toxic chemicals from the lysosome digest and destroy the pathogen
• Indigestible material is released through exocytosis

Question 24

List the main phagocytes in the immune response

Answer 24

• Macrophages, neutrophils and dendritic cells

Question 25

Compare the roles of macrophages, neutrophils and dendritic cells

Answer 25

• Macrophages eliminate pathogens through phagocytosis
  
  • They are also antigen presenting cells that activate the adaptive immune response
  • Found in tissues
• Neutrophils eliminate pathogens through phagocytosis
  
  • Most abundant and are the first to arrive at an infection site
  • Found in blood
• Dendritic cells eliminate pathogens through phagocytosis
  
  • They are also antigen presenting cells that activate the adaptive immune response
  • Found in tissues
  • Occupy and patrol the skin and mucosal surfaces

NOTE: Neutrophils are phagocytes, NOT antigen presenting cells.

Question 26

Describe the role of eosinophils and how they perform this role

Answer 26

• Eosinophils destroy larger pathogens such as parasites that are too large to be engulfed via phagocytosis
• They attack via degranulation (releasing cytotoxic chemicals)
• Found in the respiratory, gastrointestinal and urinary tracts
• They are also involved in the allergic process

Question 27

Describe the role of natural killer cells and explain how they destroy virally-infected cells

Answer 27

• Natural killer cells kill virus-infected cells through apoptosis, destroying both the cell and the virus it contains (kills pathogens once they have entered a cell, not the pathogen alone)
• The NK cell releases perforin and protease enzymes
• Perforin creates pores in the plasma membrane of the target cell allowing protease to enter
• Protease enzymes induce apoptosis
• The infected cell is eliminated
• Found in blood/lymph

Question 28

State where mast cells are located and what they contain

Answer 28

• Mast cells are found in tissue beneath the surface of the skin, near vessels and in the respiratory system
• They contain histamine, cytokines and heparin

Question 29

Describe the role of mast cells

Answer 29

• Mast cells are vital in the inflammatory response
• They release histamines causing permeability and vasodilation in blood vessels
• They release cytokines which attracts other immune cells to help destroy the pathogen

Question 30

Define permeability and vasodilation in terms of blood vessels

Answer 30

• Permeability → causes capillaries to be leaky
  
  • Protein-rich fluid (exudate) can escape from capillaries to the infected region
  • The exudate causes swelling, putting pressure on surrounding tissues and stimulating nerve endings which causes pain
• Vasodilation → causes capillaries to be wider
  
  • Increases blood flow to damaged area resulting in heat production and redness

Question 31

Describe complement proteins

Answer 31

• Complement proteins are proteins that assist other innate immune cells to destroy pathogens
• They are initially inactive but are activated when they make direct contact with antigens
• Part of the innate (non-specific) immune response

NOTE: Complement proteins are effective on all pathogens, including viruses.

Question 32

Describe how complement proteins can help to remove pathogens

Answer 32

• Lysis → complement proteins interact to form a membrane-attack complex on the plasma membrane of the pathogen
  
  • The MAC creates a pore in the plasma membrane causing the pathogen to swell and burst
• Opsonisation → complement proteins coat the surface of pathogens allowing phagocytes to more easily bind to the pathogen
  
  • Phagocytes have receptors that are complementary to those on the complement proteins (higher likelihood of phagocytosis)
• Chemotaxis → complement proteins diffuse from the pathogen and attracts immune cells to the site of infection

NOTE: Complement proteins can also coat bacteria to prevent binary fission (replication).

Question 33

State what interferons are

Answer 33

• Interferons are a subgroup of cytokines (signalling proteins) that are made and released by virally infected cells or host cells
• They interfere with viral replication
• Part of the innate immune response

Question 34

State 4 effects that interferons can have

Answer 34

• Activate immune cells such as NK cells
• Signals neighbouring infected cells to undergo apoptosis
• Signals neighbouring uninfected cells to reduce protein synthesis
• Signals change in plasma membrane to prevent further entry of the virus

Question 35

Describe the 3 stages of inflammation (VCR), including the role of various cells and proteins

Answer 35

• Vascular stage involving blood cells
  
  • Mast cells release histamines causing permeability and vasodilation in cappilaries (blood vessels)
• Cellular stage involving immune cells
  
  • Cytokines released by mast cells attract phagocytes such as macrophages and dendritic cells
  • Complement proteins opsonise the pathogen allowing phagocytosis to occur
  • Platelets travel to the site to block the wound (clotting)
  • Pus (consisting of dead tissue and pathogens) indicates that this stage is occuring
• Resolution stage when the antigen is removed
  
  • Occurs when tissue is returned to its normal state and the infection is under control
  • If the resolution stage is not reached, chronic inflammation may occur

Question 36

Outline the process that leads to a sore and inflamed throat

Answer 36

• Upon cell destruction in the throat, chemicals are released that cause increased membrane permeability and swelling
• Vasodilation allows more immune cells to move to the site
• Mast cells release histamines, which causes inflammation

Question 37

Outline how antigen presenting cells present antigens

Answer 37

• An antigen is enfgulfed by a dendritic cell or macrophage (antigen presenting cells)
• The antigen is destroyed and fragments are presented on MHC-II molecules
• The APC presents the antigen to specific helper T cells

Question 38

State where antigen presentation occurs

Answer 38

• Lymph nodes

Question 39

Explain the role of T helper cells

Answer 39

• They are activated by antigen-presenting cells that present antigens on their MHC II markers
• Once active, they clone and secrete cytokines (interleukins) which activates either the humoral adaptive immune response (B cells) or the cell-mediated adaptive immune response (cytotoxic T cells)

Question 40

Explain the role of cytotoxic T cells

Answer 40

• Upon the release of cytokines from helper T cells, cytotoxic T cells proliferate (multiply), producing activated cytotoxic T cells and memory T cells via clonal selection and expansion
• Cytotoxic T cells destroy the target cell through apoptosis

Question 41

Outline the function of the lymphatic system

Answer 41

• The lympathic system acts as a transport network
• Allows for antigen recognition
• Involved in the production and maturation of immune cells
• The removal of excess fluids from body tissues
• Absorption and transportation of fatty acids to the digestive system

Question 42

List and state the function of the primary lymphoid organs

Answer 42

• Primary lymphoid organs are where immune cells are produced and mature
• Bone marrow is the site in which all immune cells originate (including red blood cells and T cells) and where B cells mature
• The thymus is where T cells mature after being released from the bone marrow

Question 43

List and state the function of the secondary lymphoid organs

Answer 43

• Secondary lymphoid organs are where immune cells are activated by meeting antigens and where immune responses occur
• The spleen filters blood clearing it from bacteria, viruses or worn-out red blood cells
• Lymph nodes are the site for antigen recognition

Question 44

Describe both the structure and function of lymph nodes

Answer 44

• Lymph nodes are small bean shaped structures located in the armpits, neck, groin and abdomen
• Composed of lymphoid tissue with clusters of lymphocytes residing within the lymphatic system
• They are sites where antigen presentation occurs and activates T and B cells
• Swelling due to infection occurs because the number of B and T cells in the lymph nodes increase
• Valves in lymph nodes prevent the backward flow of lymph fluid

Question 45

Describe how the lympathic system acts as a transport network for immune cells

Answer 45

• Immune cells can travel around the body through the lymph

Question 46

Describe the key features of the adaptive immune response

Answer 46

• T and B cells (special white blood cells known as lymphocytes)
• Antibodies, also known as immunoglobulins (special antigen-binding proteins)
• Secondary lymphoid organs where B and T cells meet foreign antigens and are activated, and where adaptive immune responses occur

Question 47

Define clonal expansion and clonal selection

Answer 47

• Clonal expansion is when activated cells divide into many identical copies (clones)
• Clonal selection is when a cell binds with its specific antigen

Question 48

Explain adaptive immunity

Answer 48

• Adaptive immunity involves a specific response against a specific pathogen where memory is retained for future infection
• Usually only required if an infection is not cleared by the innate response
• Specificity refers to adaptive immune cells (B and T cells) having unique receptors that recognise specific antigens
• Immunological memory refers to the ability for the immune system to remember antigens from previous infections which enables a stronger and more rapid immune response upon reinfection with the same antigen

Question 49

Outline the humoral and cell-mediated parts of the adaptive immune response

Answer 49

• Humoral immunity is a response that occurs in body fluids
  
  • It involves the actions of B lymphocytes and their antibodies
• Cell mediated immunity is a response that kills body cells
  
  • Involves the actions of cytotoxic T cells
• Both produce memory

Question 50

Outline how the cell-mediated response removes body cells infected by viruses (or cancerous)

Answer 50

• Upon antigen recognition, helper T cells clone and secrete cytokines
• Cytotoxic T cells proliferate producing activated cytotoxic T cells and memory T cells through clonal selection and expansion
• Cytotoxic T cells destroy the cells through apoptosis

Question 51

Outline how cytotoxic T cells attack antigens

Answer 51

• Cytotoxic T cells release perforin that enters a target cell and create pores in its plasma membrane
• Cytotoxic T cells also release protease enzymes that enter the infected cell via the pore and initiate apoptosis
• The cytotoxic T cells are then free to attack other infected cells that display the same foreign antigen

NOTE: Apoptosis by cytotoxic T cells, unlike NK cells, are specific for certain antigens and form memory.

Question 52

Describe the key components and functions of the humoral adaptive response

Answer 52

• B cells are activated by binding to pathogens or cytokines secreted by T helper cells
  
  • Some become memory cells (stored in lymphatic system)
  • Some become plasma cells (floats around the bloodstream)
• Plasma cells make antibodies
  
  • Antibodies work against the pathogen (PIANO)

Question 53

Outline how the humoral immune response removes extracellular pathogens from the body fluids

Answer 53

• B lymphocytes (cells) are covered in specific receptors and are activated by T helper cells
• When activated by a T helper cell, B cells replicate (clonal expansion) producing plasma cells and memory cells
• The plasma cells make antibodies which can opsonise or agglutinate pathogens

Question 54

Describe the difference between memory cells and plasma cells

Answer 54

• Memory cells
  
  • Inactive
  • Lifelong
  • Produce rapid and enhanced immune responses upon secondary exposure to a previously identified antigen
• Plasma cells
  
  • Active
  • Tend to live shorter than memory cells
  • Gives protection by producing antibodies
  • Each type of plasma produces one specific type of antibody

Question 55

Outline the process that leads to the production of antibodies

Answer 55

• When a pathogen comes into contact with a B cell, the B cell divides and clones to produce memory and plasma cells
• Plasma cells produce antibodies that act against the pathogen

Question 56

Draw and label the structure of an antibody

Answer 56

• Diagram should include
  
  • 2 heavy and 2 light polypeptide chains
  • Variable and constant region
  • 2 identical specific antigen-binding sites

NOTE: Antibodies have a quaternary structure. They are composed of 4 polypeptide chains that are held together by disulphide bonds.

Question 57

Explain what creates the difference in variable regions for an antibody

Answer 57

• Antigenic variation
• The shuffling of nucleotides that code for variable regions create different combinations
• These combinations code for varying protein structures

Question 58

Describe the effects of antibodies (PIANO)

Answer 58

• Precipitation → antibodies bind to soluble agents making them insoluble to create a solid that is more visible to the immune system (due to cross-linking between antigens and antibodies)
• Inflammation → antibodies initiate the release of histamine by binding to mast cells
• Agglutination → antibodies bind to antigens forming antigen-antibody complexes causing them to clump together and become more visible to the immune system
• Neutralisation → antibodies bind to antigens and block their receptors so that the pathogens cannot attach to healthy body cells and infect them
• Opsinisation → antibodies bind to antigens to make them more susceptible to phagocytosis

NOTE: Antibodies do not directly eliminate pathogens.

Question 59

Describe 3 ways that antibodies can neutralise pathogens

Answer 59

• Bind to antigens to stop them from functioning
• Coat the surface of antigens to make them more susceptible to phagocytosis (opsinisation)
• Bind to antigens to initiate lysis by complement proteins

Question 60

Describe the difference between natural and artificial immunity

Answer 60

• Natural immunity is a form of immunity that occurs naturally without deliberate intervention
• Artificial immunity is a form of immunity that is created via deliberate intervention or exposure to an antigen

Question 61

Describe the differences between active and passive immunity

Answer 61

• Active immunity involves antibodies being produced by an individual’s own immune system in response to exposure to a particular antigen
  
  • Secondary response is enhanced due to memory, long lasting, more effective, immunity is developed over weeks
• Passive immunity involves antibodies being acquired from an external source
  
  • No immunological memory, short lasting, less effective, immunity is immediate

Question 62

State ways in which natural immunity can develop

Answer 62

• Antibodies naturally produced in response to catching a cold
• Mother-child (breastfeeding)
• Contracting chicken pox via “chickenpox parties”

Question 63

State 2 ways in which artificial immunity can develop

Answer 63

• Vaccination (deliberate exposure to a pathogen to produce antibodies)
• Injection of antibodies

Question 64

State 2 ways in which active immunity can develop

Answer 64

• Vaccines
• Infection

Question 65

State 2 ways in which passive immunity can develop

Answer 65

• Mother-child
• Injection of antibodies

NOTE: No memory cells are produced due to a lack of exposure to the antigen.

Question 66

State the function of ducts leading to and leading from the lymph nodes

Answer 66

• Ducts leading to the lymph nodes
  
  • Carry APCs that display antigens on the MHC-II molecules
  • Carry free-floating antigens
• Ducts leading from the lymph nodes
  
  • Carry specific cytotoxic T cells
  • Carry specific helper T cells
  • Carry specific antibodies

NOTE: From NEAP 2023 exam.

Question 67

List the changes that occur during an inflammatory response

Answer 67

• Increased…
• Mast cell activation
• Histamine release
• Blood flow to the area
• Swelling
• Blood vessel dilation
• Clotting factors

Question 68

Distinguish between T memory and B memory cells

Answer 68

• T memory cells proliferate into T helper cells and cytotoxic T cells when the body is exposed to a previously identified pathogen, mounting a quicker and stronger immune response
• B memory cells will rapidly divide and form new antibody-producing plasma cells

NOTE: T memory cells are involved in cell-mediated immunity while B memory cells are involved in humoral immunity.