Chapter Answers

1. Explain the immunological mechanisms that are responsible for allergic reactions

Answer: The allergen binds to immunoglobulin E (IgE) antibodies on the surface of mast cells and trigger a hypersensitive immune reaction specific for environmental antigens such as pollen, house dust, foods such as shell fish and nuts, and chemicals such as the antibiotic penicillin.

T-helper lymphocytes (TH2 cells) are involved in the production of IgE antibodies that bind to mast cell on exposure to the antigen. This is called sensitization and is the reason why this type of reaction occurs so rapidly on subsequent exposure to the antigen. The mast cell releases mediators such as histamine which causes bronchial smooth muscle contraction, vasodilation, increased vascular permeability and increased mucous production by nasal and bronchial glands. Histamine binds to endothelial cells leading to increased vascular permeability and leakage of plasma into the tissues resulting in oedema. Basophils are white blood cells that can be found in the circulation and are also involved in type 1 hypersensitivity reactions in a similar way to mast cells.

2. Explain why it is important that people with a known risk of anaphylactic shock need to carry an epinephrine ‘pen’ with them at all times.

Answer: Anaphylactic shock results from systemic vasodilation causing decreased blood pressure and shock along with constriction of the upper airways and laryngeal oedema resulting in difficulty in breathing. Anaphylactic reaction can be life-threatening and administration of epinephrine using an Epi-pen is required to reverse the bronchoconstriction and vasodilation and improve cardiac output.

3. Discuss the type of reaction triggered by a Mantoux test and why it is used.

Answer: This is a delayed-type hypersensitivity (DTH) involving the activation of CD4+ helper T cells and the secretion of cytokines that promote an inflammatory reaction. This reaction occurs in people who have had previous exposure to the antigen and are therefore sensitised. The reaction is delayed as it develops 24–72 hours after exposure to the antigen and involves the activation of Th1 cells that are ultimately responsible for the reaction. Cytokines, macrophages, fibroblasts and leucocytes are also involved in the process. A Mantoux test is used to identify whether a person has been previously exposed to the tuberculin antigen. A small amount of the tuberculin antigen is injected under the skin and within 4 hours neutrophils accumulate around the injection site. Within 12 hours T cells infiltrate the area and blood vessels become permeable. Fibrinogen leaks from the plasma into the surrounding tissue and is converted into fibrin. The surrounding tissue becomes oedmatous and appears firm (due to the fibrin) and inflamed. This indicates previous exposure to the tuberculosis mycobacterium.

4. Describe four clinical features of SLE.

Answer: 

Clinical presentations of at least four indicate SLE (Petri et al., 2012)

  • Facial rash confined to cheeks
  • Discoid rash (raised patches, scaling)
  • Photosensitivity
  • Oral or nasopharyngeal ulcers
  • Nonerosive arthritis of a least two peripheral joints
  • Serotitis (inflammation of the membranes of lung or heart)7.     Renal disorders
  • Neurological disorders
  • Haematological disorders
  • Immunological disorders
  • Presence of antinuclear antibody (ANA)

5. Identify how HIV is transmitted.

Answer: HIV is transmitted through body fluids that contain the virus via three major routes, sexual contact, inoculation with infected blood and mother to baby transmission.

  • Sexual contact is the most frequent mode of transmission where infected semen or vaginal fluid is exchanged and eventually gains entry to the bloodstream, for example through the mucosal lining of the vagina, anal mucosa, wounds or sores. Anti-retroviral treatment (ART) in a HIV-positive person can reduce the risk of transmitting the virus to an uninfected sexual partner (WHO, 2015)
  • Inoculation of infected blood or blood products is also a mode of transmission, particularly where needles are shared by intravenous drug users. The risk of transmission through blood transfusion is small as blood and blood products are screened for HIV. However, there is still a small risk due to a window where the infection is not detected but due to advanced testing of blood donors the risk is 1 in 2 million.
  • Mother to baby transmission occurs most frequently in utero or during childbirth or during breastfeeding.

6. Describe how HIV causes destruction of the immune system. 

Answer: HIV is a retrovirus with glycoproteins on its surface. Glycoprotein gp120 binds to the CD4 molecules on the surface of T lymphocytes and promotes a secondary gp120 binding to a chemokine receptor on the cell surface. This induces gp41 to insert into the cell membrane enabling the viral membrane to fuse with the target cell. Once inside the cell RNA undergoes reverse transcription to become viral DNA This leads to the production of virons which are released from the plasma membrane of the host cell causing destruction of the T cells. This is a cycle that ultimately results in the destruction of the immune system.

7. Outline the virulence factors that a microorganism needs to cause infectious disease.

Answer: 

  • Microorganisms that cause infectious disease need to be capable of avoiding the immune system.
  • They need to spread from one infected person to another. This is known as communicability. Certain microorganisms have high communicability and spread easily, making them difficult to control.
  • They also need to be able to gain entry to the host and multiply (infectivity).
  • They need to be able to cause disease (pathogenicity).

8. Describe the stages of the sexually transmitted infection syphilis.

Answer: 

  • The primary stage occurs several weeks after infection and is characterised by a primary lesion called a chancre at the point of contact on the skin or mucous. This is a painless firm ulcerated nodule. The infection is highly contagious.
  • The second stage or secondary syphilis occurs when the spirochetes have entered the general circulation and there is lymphadenopathy. A widespread maculopapular reddish rash appears on the palms of the hand and the soles of the feet. Other general signs of infection, such as fever, sore throat, loss of appetite and inflamed eyes are common.
  • Asymptomatic latent phase which may last for many years.
  • Tertiary stage where the infection progresses and there is the formation of necrotic fibrous lesions known as gummas which can lead to aortic aneurysms, bone destruction and damage to the central nervous system, leading to blindness, dementia and sensory/motor loss.

9. What are the risk factors for developing meningitis?

Answer: Risk factors associated with developing meningeal infection include base of skull fractures, otitis media, sinusitis neurosurgery, systemic sepsis and a compromised immune system. Neisseria meningitidis is often carried in the nasopharynx of an asymptomatic carrier and is spread by respiratory droplet transmission, through kissing, coughing, sneezing or sharing utensils, food and drink. It is often epidemic in environments where there is close contact between people, such as educational institutions or where young people reside together.

10. Outline the clinical signs of systemic inflammatory response syndrome (SIRS).

Answer:  

  • Respiratory rate >20 breaths/min
  • Heart rate> 90 beats/min
  • Body temperature >38OC or <36OC
  • White blood cell count >12,000 cells/mm3 or <4000/mm3

11. Explain how sepsis can lead to multiple organ dysfunction (MODs)

Answer: Widespread vasodilation caused partly by NO being further released from damaged endothelial cells. This leads to hypotension, and compensatory tachycardia with increased cardiac output leading to septic shock. Vascular leakage leads to tissue oedema. Decrease in the production of anti-coagulant factors and an increase in pro-coagulation factors. Tissue perfusion is reduced. Formation of thrombi in the microcirculation. Coagulation factors and platelets are consumed resulting in bleeding and haemorrhage. This derangement of coagulation is known as disseminated intravascular coagulation (DIC). Poor tissue perfusion leads to cellular and tissue hypoxia eventually progressing to septic shock.

Septic shock can progress to multiple organ dysfunction syndrome (MODS) where two or more organ systems fail due to the overwhelming uncontrolled inflammatory response. Sepsis can progress rapidly to

12. Explain the six steps of treatment once sepsis is suspected.

Answer: 

  1. Administer oxygen: improve oxygen content to the blood and therefore its delivery to the tissues

  2. Take blood cultures: help identify pathogens to determine likely source and guide antimicrobial therapy

  3. Give IV antibiotics: control underlying infection, removing the trigger for immune over reaction

  4. Give IV fluids: improve preload to the heart by correcting hypovolaemia, improving cardiac output and BP

  5. Check serial lactate: High lactate indicates hypoperfusion. Response of lactate helps guide resuscitation

  6. Measure urine output: Urine output falls if the patient is hypovolaemia, also provides an indicator of adequate cardiac output.

13. Explain how microorganisms become resistant to antimicrobial drugs.

Answer: There are several ways bacteria can develop resistance to antibiotics.

  • Gram-negative bacteria produce enzymes that destroy the β-lactam component of penicillin-type antibiotics.
  • A reduction in permeability, including closure of porin channels in the cell wall, prevents entry of the antibiotic into the cell
  • The creation of a modified target so that the antibiotic no longer recognises its target receptor.
  • Export mechanism to actively pump the antibiotic out of the cell. 

14. Identify what steps can be taken to reduce microbial resistance.

Answer: 

  • Prudent use of antimicrobials, for example, avoidance of prescribing antibiotics for viral infection for which they have no effect
  • Effective infection control measures to reduce spread
  • Reduced use of antibiotics in farm animals
  • Screening and isolation if infected patient is in hospital