* Body defence mechanism (Ch 24)

Question 1

First line of defense: Physical barriers

Answer 1

1. Skin
2. Hair at nostril
3. Musus secreting cells and ciliated epithelial cells of respiratory tract
   - Mucus secreting cells secrete mucus to trap germs / pathogens / bacteria from incoming air. Beating action of cilia sweeps the trapped pathogens away to the pharynx for swallowing to stomach or coughing.
4. Blood clotting
   - Damaged cells and blood platelets release clotting factors, which brings about the formation of an activated enzyme. The activated enzyme catalyses the conversion of fibrinogen into fibrin, which forms a network entangling red blood cells, white blood cells, platelets and pathogens to form clot blocking up the injured sites of the blood vessel
   - Prevent further bleeding
   - Prevent invasion of pathogens through the wound

Question 2

First line of defence: Chemical barriers

Answer 2

• Tear: contains lysozyme to kill the pathogens, helps wash away the pathogens from the conjunctiva
• Saliva: contains lysozyme to kill the pathogens
• Sebum: contains antiseptic which inhibit the growth of pathogens
• Sweat
• Gastric juice: contains hydrochloric acid which kills pathogens entered the stomach with food
• Vaginal secretions: contains acidic substances to inhibit the growth of pathogens in vagina

Question 3

Second line of defence

Answer 3

1. Phagocytosis
   - When the skin is damaged and pathogens entered the body, phagocytes squeeze through the capillary wall and engulf the pathogens
2. Inflammatory response
   - Red: arterioles of the tissue with inflammatory response dilate, increasing blood flow to the tissue and makes it red
   - Hot: high metabolic activity, produces more heat / caused by extra blood flow to inflamed area and blood carries heat
   - Swelling: permeability of capillary wall increases, increasing the rate of formation of tissue fluid. When the rate of tissue fluid formation is higher than the rate of tissue fluid draining away, accumulation of tissue fluid at the infected site causes swelling
   - Pain: accumulation of tissue fluid presses against nerve endings, stimulating pain receptors and gives pain sensation

• Increased blood flow and permeability of blood capillaries allow more phagocytes to reach the infected area to engulf and kill invading pathogens

3. Antimicrobial proteins
4. Fever
   - Fever increases body temperatures to activate proteins involving in body defence and suppress microbial growth and propagation

Question 4

Sub-cellular structure of phagocytes for phagocytosis

Answer 4

• Lysozyme: to kill the pathogens
• Mitochondria: provide energy for synthesis of protein
• Rough endoplasmic reticulum: synthesis of lysozyme
• Glycoprotein on cell membrane: recognize pathogen
• Fluid nature of cell membrane: to engulf pathogens

Question 5

Antigen

Answer 5

• Some surface molecules with specific shape, which are recognized as foreign and stimulate an immune response

Question 6

Lymphocytes production, maturation, storage

Answer 6

B cells
- Production: bone marrow
- Maturation: bone marrow

T cells
- Production: bone marrow
- Maturation: thymus

Both storage: lymph nodes and spleen

Question 7

B cell immune response

Answer 7

• When antigen on the surface of pathogen bind to the receptor site of B cell, the binding stimulates the B cell to proliferate and differentiate into plasma cells. The plasma cell then form a large number of antibodies to combat against the pathogen

*Plasma cells have large number of rough ER and mitochondria for active production of antibodies
*No B cells without helper T cells

• Some B cells proliferate and differentiate into memory B cells when they encounter the antigen of the pathogen for the first time. When the same pathogen invades the body, it will be recognized by the memory B cells, which will proliferate and differentiate into plasma cells rapidly, resulting in the production of large number of antibodies within a short time to act against the pathogen

Question 8

Action of antibodies

Answer 8

• Antibodies indirectly cause lysis of pathogens through binding on them. This attracts certain proteins in blood to cause lysis of the pathogens
• Antibodies attach to the antigens on the surface of pathogens. Antibodies bind to several pathogens together as a clump to enhance the phagocytosis by phagocyte
• Act as antitoxins which binds to the toxins secreted by bacteria. This makes the toxins harmless

Question 9

T cells

Answer 9

• Antigens of pathogens stimulates T cells to proliferate and differentiate
• Killer T cells kills the infected cells directly
• Helper T cells secrete lymphokines to stimulate B cells to proliferate and differentiate into plasma cells to produce antibodies / activate T cells to proliferate and differentiate into killer T cells and memory T cells / activate phagocyte to engulf pathogens
• Memory T cells: when the same type of antigen invades the body again, memory T cell formed during the first exposure to the antigen will proliferate and differentiate rapidly to form a large number of killer T cells within a short period to time to kill the infected cells

Question 10

Relationship between phagocytosis and specific immune responses

Answer 10

• After phagocytosis, the antigens of pathogens will be broken down intracellularly. The antigens will be presented to the lymphocytes for recognition. This elicits specific immune responses
• Activated helper T cells liberate chemicals to activate phagocytosis.
• Antibodies cause clumping of pathogens which make them more easily to be phagocytosed

Question 11

Limitations of body defence mechanism

Answer 11

• Mosquitoes with piercing mouthparts break the integrity of the skin
• Pathogens can mutate, thus not recognized by memory B and T cells to elicit secondary response
• Delayed response allows pathogens to multiply
• Hyperstimulation of immune system kill our own body cells

Question 12

Primary vs Secondary response

Answer 12

Primary response: quicker decrease in level of antibodies
- Antibodies are used up
- Plasma cells degenerate

Question 13

Immunity

Answer 13

Active immunity
- Antibodies produced by oneself
- Memory cells produced
- Natural: recovery from a disease
- Artificial: vaccination

Passive immunity
- Antibodies received from others
- Natural: antibodies in maternal blood pass through the placenta and enter into the foetal blood / maternal antibodies in mother’s milk pass to the newborn through breast feeding
- Artificial: injection of antibodies

Question 14

Principle of vaccination

Answer 14

• During the first injection, as the vaccine contains antigens, it stimulate B cells to proliferate and differentiate into memory B cells. When the same antigen invades the body again, the antigen will be recognized by memory B cells which proliferate and differentiate into plasma cells rapidly, resulting in the production of a large amount of antibodies within a short period of time to act against the pathogen, leading to a sharp rise in the protection against the infection

Question 15

Side effects of vaccination

Answer 15

• Pain in injection site
• Headache
• Fatigue

Question 16

Advantages and disadvantages of using weakened pathogen to make vaccine

Answer 16

Advantages
- Live attenuated vaccines are great at teaching the immune system how to fight off a particular pathogen because they are closest to a natural infection
- One or two doses are required for life-long immunity for most diseases
- Easier to create for certain pathogens compared to other methods

Disadvantages
- Weakened pathogens may have the ability to gain control of their natural environment again and cause disease
- Vaccines cannot be administered to people with weakened immune systems

Question 17

Events lead to the production of viral spike proteins in human cells after injection of mRNA vaccine

Answer 17

• The mRNA in the vaccine coding for the viral protein binds to a ribosome. Specific amino acids are carried to the ribosome by tRNA. The anticodons of tRNA molecules bind to the complementary codons on mRNA. Adjacent amino acids are joined together by a peptide bond, forming the polypeptide which then coils and folds to form viral spike protein.

Question 18

Why viral spike protein can stimulate the antibody production to act against COVID-19 virus

Answer 18

• The viral spike protein is recognized by B cells in the body as foreign antigens. B cells are stimulated to proliferate and differentiate into memory B cells to the virus. When the COVID-19 virus invades the body, memory B cells recognize the same antigens, which then proliferate and differentiate quickly into plasma cells, which produce a large amount of antibody to act against the virus within a short period of time.

Question 19

Do mRNA vaccines cause people to develop COVID-19

Answer 19

• No. mRNA vaccines only contain mRNA coding for viral spike proteins, they do not use live virus that causes COVID-19

Question 20

Do mRNA vaccines affect or interact with human DNA

Answer 20

• No. mRNA never enters the nucleus of the cell where DNA is located. Translation of mRNA only occurs in cytoplasm. mRNA of vaccine is short-lived

Question 21

Problems arising from immune system

Answer 21

1. Blood transfusion
   - Antibodies in blood plasma bind to the foreign red blood cells and make them clump together
2. Organ transplant
   - Antigens of new organ will be recognized as foreign by immune system, T cells proliferate and differentiate into killer T cells / helper T cells…, leading to the destruction of new organs being transplanted
   - Immunosuppressive drugs taken by recipient to inhibit antibody production and suppress responses of T cells. The patient is susceptible to infections.
3. Allergy
   - Over-reaction of immune system
4. Autoimmune disorders
   - Our immune system attack our own cells

Question 22

How antihistamine relieve the symptoms of allergy

Answer 22

• As the shape of antihistamine molecule is similar to that of histamine molecule, antihistamine blocks the histamine receptor site and prevents histamine from combining with the receptor

Question 23

COVID-19 symptoms, transmission

Answer 23

Symptoms
- fever, dry cough, fatigue, loss of taste

Transmission
- By respiratory droplets or contact