Infectious Diseases

Question 1

Why is HIV described as a retrovirus?

Answer 1

• HIV is an enveloped virus that contain two identical molecules of single-stranded RNA
• It contains reverse transcriptase, which uses its single-stranded RNA as template for synthesis of double-stranded RNA
• It contains integrase which catalyses the integration of the double-stranded viral RNA into host cell DNA as provirus

Question 2

Main steps in antigen presentation by APC

Answer 2

• Bacterial cell is engulfed by the APC through the extension of pseudopodia during phagocytosis
• Membrane of phagocytic vesicle fuses with lysosomal membrane
• Hydrolytic enzymes in the lysosome digest the pathogen and pathogenic proteins into peptides
• The peptides bind to MHC molecules to form peptide-MHC complex on the cell surface of APCs
• Remaining fragments of pathogen are released out of the APCs by exocytosis

Question 3

How is genetic variation generated by somatic recombination and meiosis different?

Answer 3

• Somatic recombination resulted in shortening of the chromosome due to deletion of genes from the VDJ domains, whereas crossing over and independent assortment in meiosis can still result in chromosomes of the same length
• Somatic recombination takes place within 1 chromosome, whereas meiosis requires 2 homologous chromosomes to occur

Question 4

Outline the role of antibodies in eliminating dengue virus from the blood

Answer 4

• Antibodies neutralise dengue virus in the blood
• Antibodies bind to viral particles to prevent them from interacting with the host cell receptors, thus preventing viral entry into host cells
• Agglutination occurs when dengue virus bound by antibodies are concentrated, thus lesser infectious units, so easier to clear dengue virus

Question 5

Outline the components of the non-specific immune system in mammals

Answer 5

• Physical barriers such as the skin and mucous-covered epithelial tissues
• Chemical barriers such as antibacterial enzymes in secretions (eg. hydrochloric acid in stomach)
• Cellular components such as phagocytes like macrophages, dendritic cells and neutrophils that engulf pathogens via phagocytosis
• Macrophages release cytokines and chemokines involved in inflammatory response as they increase permeability of blood vessels and recruit more phagocytic cells

Question 6

Outline role of T helper cells in fighting infection

Answer 6

• Binding of T cell receptor to peptide-MHC complex on cell surface of B lymphocytes and secretion of cytokines by helper T cells
• Helps in activation of naive B lymphocytes to undergo clonal expansion and differentiation to produce plasma cells and memory cells
• Secretion of cytokines by helper T cells helps in activation of naive CD8 T cells to differentiate into cytotoxic T cells
• Secretion of cytokines by helper T cells also help in class switching in activated B lymphocytes to produce antibodies of different classes

Question 7

Explain how babies are born with some short term antibodies circulating in their bloodstream

Answer 7

• The antibodies found in newborns come from their mother
• They are passed on to the babies via the placenta in the foetal stage
• Providing natural passive immunity to the babies
• Antibodies are broken down in the body and not replaced therefore immunity is short-lived (absence of memory B and T cells in the newborn babies)

Question 8

Describe the roles of T cytotoxic cells in the cell mediated immune response

Answer 8

• Releases perforins which form pores in infected cell membrane
• Granzymes enter infected cells via endocytosis to induce apoptosis
• Release cytokines which inhibits viral replication and induces expression of MHC molecule in infected cells

Question 9

Explain how action of phagocytes is assisted by the humoral immune response

Answer 9

• Neutralisation of bacterial toxin and virus particles by antibodies binding to them
• Opsonisation of bacterial cells where the antibodies bind to antigens on the bacterial cell
• Constant region of antibody binds to Fc receptors on phagocytes facilitating phagocytosis of antigen-antibody complex

Question 10

Explain how response to a second infection by same pathogen differs from the first

Answer 10

• Secondary response is more rapid and effective than the primary response and pathogen is removed faster
• Memory cells have been produced during the first infection
• They can be reactivated more quickly than naive lymphocytes and there are now many more cells that are specific for the pathogen
• Faster production of antibodies

Question 11

Explain the relationship of the molecular structure of antibodies to their functions, using lgG as an example

Answer 11

• Large quaternary protein composed of four polypeptide chains
• It consists of two identical heavy chains and two identical light chains linked by disulfide bonds
• Heavy and light chains each have variable and constant regions
• Variable regions of heavy and light chains at the amino terminus form the antigen-binding site which determines the antigen specificity of the antibody
• As the antibody has two identical variable regions, there are two identical antigen-binding sites, allowing the antibody to bind to two identical antigens simultaneously

Question 12

Explain the relationship of the molecular structure of antibodies to their functions (using lgG as an example)

Answer 12

• Large quaternary protein composed of four polypeptide chains
• It consists of two identical heavy chains and two identical light chains linked by disulfide bonds
• Heavy and light chains each have variable and constant regions
• Variable regions of heavy and light chains at the amino terminus form antigen-binding site which determines the antigen specificity of the antibody
• As the antibody has two identical variable regions, there are two identical antigen-binding sites
• Allowing the antibody to bind to two identical antigens simultaneously, therefore increasing total strength of interaction
• Constant regions of heavy and light chains at the carboxyl terminus determine the function of the antibody
• The flexible stretch of polypeptide chain joining the Fab and Fc fragments is known as the hinge region
• Allows flexibility of antibody to bind to multiple antigens

Question 13

Explain circumstances which lead to the production of antibodies

Answer 13

• Naive B lymphocytes circulate the blood and lymph
• They possess a repertoire of antigen receptors, each cell having receptors that is specific for a particular antigen
• The B cell receptor of a naive B lymphocytes binds to an intact antigen in the blood or lymph
• Antigen is taken up via endocytosis and digested into short peptides
• The peptide is then presented on the MHC molecule forming a peptide-MHC complex
• T cell receptor of the helper T cell binds to the peptide-MHC complex on the B lymphocytes
• Resulting in the secretion of cytokines by helper T cells for the activation of naive B lymphocytes
• Upon binding to its antigen and interacting with helper T cell, the B lymphocyte is activated to proliferate and produce many identical progenies, a process known as clonal expansion
• The progeny then differentiate into plasma cells and memory B lymphocytes
• Plasma cells then secrete the antibodies via exocytosis
• These antibodies have the same antigen specificity as the B-cell receptor found on the naive B lymphocyte previously

Question 14

Explain how vaccination gives protection against disease

Answer 14

• Vaccination is the intentional administration of inactivated pathogens, live attenuated pathogens or its antigens into the body
• Vaccines induce a specific adaptive immune response that protects the individual against later exposure to the pathogen due to the production of memory cells
• During the primary response, a particular surface antigen of the pathogen is still retained and is recognised by a specific B lymphocyte
• Activated B lymphocyte then undergoes clonal expansion by dividing and differentiating to form memory cells and antibody-secreting plasma cells
• During the secondary response, when the individual is exposed to the same pathogen again, memory cells in a previously vaccinated individual will quickly recognise the surface antigen of the pathogen
• Reactivated memory cells undergo rapid clonal expansion and develop into antibody secreting plasma cells
• These plasma cells are then able to produce large number of antibodies which bind and neutralise the virulent pathogen to prevent them from infecting healthy host cells