Topic 11.1 Defence Against Infectious Disease

Question 1

11.1.1 Describe the process of blood clotting.

Answer 1

Clotting is a mechanism that prevents the loss of blood from broken vessels.

1. Damaged cells and platelets release chemical signals called clotting factors (prothrombin activator) which trigger a coagulation cascade:
2. Clotting factors convert the inactive prothrombin (proteins in blood) into the activated enzyme thrombin
3. Thrombin catalyses the conversion of the soluble plasma protein fibrinogen into fibrin (insoluble form)
4. Fibrin forms an insoluble mesh of fibres that trap blood cells at the site of damage
5. Clotting factors also cause platelets to become sticky, which then adhere to the damaged region to form a solid plug called a clot
6. The clot prevents further blood loss and blocks entry to foreign pathogens

Question 2

11.1.2 Outline the principle of challenge and response, clonal selection and memory cells as the basis of immunity.

Answer 2

Challenge and Response

• When body is challenged by foreign pathogen, it will respond with both non-specific and specific immune reaction
• Body is capable of recognizing invaders b/c they don’t have molecular markers that designate all body cells as “self”
• Non-specific immune cells (macrophages) present foreign antigens on their surface to lymphocytes (antigen presentation)
• These lymphocytes can then respond with production of antibodies to destroy foreign invaders

Clonal Selection

• When antigens are presented to B cells (and T_H cells) by macrophages, only the B cells with appropriate antibody will activate and clone
• The majority of B cell clones will differentiate into antibody-producing plasma cells. A minority will become memory B cells (B_M cells)
• Because pathogens may contain several antigenic determinants, several B cell clones may be activated (polyclonal activation)

Memory Cells

• Because adaptive immune response is dependent on clonal expansion to create sufficiently large amounts of antibodies, there’s a delay between initial exposure and production of antibodies
• When B cells divide and differentiate into antibody-secreting plasma cells, a small proportion of clones differentiate into memory B cells
  
  • memory B cells can remain in body for years, or lifetime
• Therefore, if second infection with same antigen occurs, memory cells allow faster and more vigorous response than initial immune response, such that symptoms of infection don’t normally appear
• Because individual doesn’t present symptoms of infection anymore, they’re said to be immune

Question 3

11.1.3 Define active and passive immunity.

Answer 3

Active immunity:

• organism responds directly to an antigen itself
• body makes antibodies specific to antigen
• memory B cells are also created to remember how to create correct antibody in case of future infection
• creates long term immunity

Passive immunity:

• acquisition of antibodies received from other organism, in which active immunity has been stimulated
• no long term immunity
• can be achieved via placenta, breastfeeding or by injection (e.g. blood transfusion)

Question 4

11.1.4 Explain antibody production.

Answer 4

Antigens stimulate immune response via production of antibodies.

1. When pathogen invades body, it’s engulfed by wandering macrophages which present antigenic fragment on its surface
2. Macrophage becomes an antigen-presenting cell, and presents antigen to helper T cells (T_H cells)
3. T_H cells bind to antigen and become activated, and in turn activate B cell with appropriate antibody for the antigen
4. B cell clones and differentiates into plasma cells and memory cells
5. Plasma cells produce high quantities of specific antibody to antigen, killing the pathogen
6. Memory cells survive in bloodstream for years
7. Upon re-exposure to antigen, memory cells initiate faster and stronger response → thus confer long-term immunity

Question 5

11.1.5 Describe the production of monoclonal antibodies and their use in diagnosis and in treatment.

Answer 5

Monoclonal Antibodies

• large quantities of a single type of antibody, produced using the following procedure:
  
  1. Antigens corresponding to desired antibody are injected into an animal
  2. B-cells producing desired antibody are extracted
  3. Tumour cells obtained from a another source (divide and grow ceaselessly)
  4. B cells fused with tumour cells, producing hybridoma cells (divide endlessly) that provide desired antibody
  5. Hybridoma cells are cultured and antibodies are extracted and purified
• Diagnosis
  
  • malaria (ELISA test)
    
    • test plate coated with antibodies that bind to antigens in malarial parasites; colour change by using more monoclonal antibodies with enzymes attached allows detection
  • HIV
  • HCG (pregnancy test)
  • blood and tissue tying for transplant compatability
• Treatments
  
  • rabies (controls virus while person’s body begins making antibodies)
  • cancer (drugs attached to monoclonal antibodies that will only bind to cancer)

Question 6

11.1.6 Explain the principle of vaccination.

Answer 6

• Based on active immunity and the production of memory cells
• Antigen (bacteria or virus) is heated or chemically treated to kill its DNA; outer coat undamaged
• Weakened or dead antigen is injected
• Immune system responds as if it were an active antigen
• Creates antibody-secreting plasma cells and memory cells
• Provides long term immunity
  
  • may need booster shot

Question 7

11.1.7 Discuss the benefits and dangers of vaccination.

Answer 7

Benefits

• Epidemics and pandemics can be prevented/some diseases completely eradicated
• “Childhood diseases” which are often more severe when contracted as an adult can be prevented
  
  • e.g. sterility in men with measles or mumps
• Death due to disease can be prevented
• Disability due to disease can be prevented
  
  • ​fetuses of pregnant women with rubella aren’t at risk of severe birth defects
• Huge cost savings: cheaper for gov’t to pay for vaccinations than caring for people who have complications

Dangers

• If vaccine isn’t “dead”, could receive disease
  
  • People with weak immune systems (HIV patients, elderly) can contract disease from someone vaccinated with a live virus
• Small % of people have serious reaction to vaccine
  
  • e.g. whooping cough vaccine may cause brain damage