O infectious disease

Question 1

explain the molecular structure of antibodies to their functions using lpG as an example

Answer 1

1. large quaternary protein composed of FOUR polypeptide chains
2. consists of two identical heavy chains and two identical light chains linked by DISULFIDE bonds
3. heavy and light chains each have variable and constant regions
4. variable regions of heavy and light chains at the AMINO TERMINUS form the antigen binding site -> allows the antibody to bind to two identical antigens simultaneously, increasing total strength of interaction
5. constant regions of heavy and light chains at the CARBOXYL terminus determine the function of the antibody -> flexible stretch of polypeptide chain joining Fab and Fc fragments is known as the hinge region which allows flexibility of antibody to bind to multiple antigens

Question 2

what leads to the production of antibodies?

Answer 2

1. naive B lymphocytes circulate the blood and lymph and possess a repertoire of B cell receptors, each cell having receptors that is specific to a particular antigen
2. the B-cell receptor of a naive B-cell binds to an intact antigen in the blood or lymph
3. antigen is taken up via endocytosis and digested into short peptides
4. the peptide is then presented on the MHC molecule forming a peptide-MHC complex
5. T-cell receptor of helper T cell binds to peptide MHC complex on B lymphocytes resulting in the secretion of cytokines by helper T cells for the activation of naive B lymphocytes

Question 3

what is the role of lymphocytes in cell-mediated immunity?

Answer 3

1. cytotoxic T cells carry out cell-mediated immune responses, killing cells that are infected with viruses or other pathogens bearing the antigen in cell-mediated immune responses
2. this is done by releasing perforin molecules which form pores in the infected cell membrane and granzymes which enter the infected cell and induce apoptosis
3. cytotoxic T cells also release cytokines which inhibits viral replication and induces expression of MHC molecules
4. naive B lymphocytes proliferate and differentiate into plasma cells which secrete antibodies that have the same antigen specificity as the B cell receptor

Question 4

what is the role of lymphocytes in humoral immunity?

Answer 4

1. antibodies provide humoral immunity by protecting against extracellular pathogens of their products
2. this occurs through neutralisation of bacterial toxin and virus particles where antibodies bind to toxins and virus particles to prevent them from interacting with host cells
3. the antibody binds to receptors expressed on macrophages and other phagocytes, facilitating phagocytosis
4. cytokines secreted by helper T cell help induce class switching to produce antibodies of different classes
5. T cell receptor of helper T cell binds to peptide-MHC complex on the B lymphocytes
6. memory cells trigger a faster, more rapid and more effective secondary response if the same antigen is encountered again

Question 5

main stages in antigen presentation

Answer 5

1. bacterial cell is engulfed by the APC through the extension of pseudopodia during phagocytosis
2. membrane of phagocytic vesicle fuses with lysomal membrane
3. hydrolytic enzymes in lysosome digest the pathogen and pathogenic proteins into peptides which bind to MHC molecules to form peptide-MHC complex
4. remaining of fragments of pathogen are released out of the APCs by exocytosis

Question 6

why are plasma cells rich in ER and mitochondria?

Answer 6

• RER is the site of synthesis of secretory proteins and are where antibodies are synthesised
• site of respiration which produces ATP for exocytosis for secretion of antibodies

Question 7

action of phagocytes by humoral immune response

Answer 7

neutralisation of bacterial toxin and virus particles by antibodies binding to them AND preventing entry into host cells + opsonisation of bacterial cells where the antibodies bind to antigens on bacterial cell