Ch3

• killed or weakened version or a subunit of a pathogen • trick the immune system as if the body was attacked by pathogens –> generation of memory B and T cells –> secondary immune response –> rapid elimination of pathogens

• safety => no risk of causing disease => few side effects • efficacy => stimulates helper T cells –> maximise the activity of immune cells through the release of cytokines –> development of memory B and T cells and eliminate pathogens by antibody and cytotoxic T cells • pathogenicity => only cause mild disease symptoms => targeted pathogens only undergo little mutation => no latency => no attack on self-immune cells

• prepared by weakened pathogens through attenuation

• growing pathogens under abnormal culture conditions –> pathogens mutate to adapt –> lose their pathogenicity • mutating virulent genes by recombinant DNA technology

• pros => retain their ability to replicate in the host –> act as intracellular Ag –> form peptide-MHC I complex –> increase both humoral and cell-mediated immune response –> lower the chance of having boosters • cons => requires refrigeration –> high production cost –> high selling price => pathogens may mutate back to pathogenic form through reversion as they are still active and living pathogens => more side-effects as they mimic real pathogens –> too effective

• whole pathogens are killed by heat, gamma irradiation or chemicals –> protect epitopes but remove pathogen’s virulence

• pros => no possibility of reversion –> safer to use => more stable than live vaccines • cons => dead microbes cannot act as intracellular Ag –> Ag not presented on MHC I –> mainly B cell response but no cytotoxic T cell response –> boosters and adjuvants are often required

• produced by recombinant DNA technology • DNA encoding the protein subunit antigen is introduced into E. coli –> expression of large amount of protein Ag –> direct inject protein as vaccine

• pros => contains only the protein Ag –> safer as there is no risk of reversion • cons => no cytotoxic T cell activation as Ab response predominates

• stimulate immune response against polysaccharide molecules in bacterial capsule

• pros => no need to use the whole bacterial pathogen –> safe • cons => only T-independent antigens involved –> B cells response only –> as there is no helper T cells response –> no memory cells

• improved by chemically linking a carbohydrate antigen to a carrier protein –> conjugate vaccine –> act as T-dependent antigen –> activation of both B and helper T cells response –> production of Ab and memory cells

• transfer a gene encoding a protein Ag of a pathogen into an attenuated viral vector –> form a recombinant vector –> infect host muscle cells –> present protein Ag both intracellularly and extracellularly

• pros => strong humoral and cell-mediated response => low risk of reversion as most of the pathogen genome is eliminated • cons => side effects due to host immune response to the recombinant vector • vector may be damaged –> vaccine becomes useless • may be life-threatening as the vector may still cause death => low stability as refrigeration is required

• plasmid DNA or RNA coding for a protein Ag of a pathogen is injected into the muscle –> host muscle cells take up DNA or RNA and express the protein Ag intracellularly and extracellularly –> present Ag via MHC I and MHCII –> increase humoral and T cells response

• pros => induce both humoral and cell-mediated immunity => customisable and easy to scale up • cons => integration of plasmid DNA in host genome may induce mutation (as the DNA will get into the host nucleus and randomly insert into the genome) => ultra-low temperature is required for the storage of mRNA vaccines

• natural active –> infection • artificial active –> vaccination • natural passive –> transfer of antibodies from mother to foetus • artificial passive –> injection of ready-made antibodies from other individuals • pros –> provide instant protection • cons –> no memory cells are produced –> short-term protection

• given during infancy to prevent childhood diseases • booster doses are usually given 2-3 times • combination vaccines • examples: => DTaP vaccine > against diphtheria, tetanus and pertussis (a = acellular) > pertussis protein combines with diphtheria and tetanus toxoids > 80% vaccinees who received all 3 doses are completely protected => MMR vaccine > live attenuated measles, mumps and rubella viruses > both humoral and cell-mediated immune response with memory B and T cells generated > highly effective –> measles, mumps and rubella are rare in developed countries

• Bacille Calmette-Guerin (BCG) –> tuberculosis • Hepatitis B –> Hepatitis B • DTaP-IPV (inactivated poliovirus) –> diphtheria, tetanus, pertussis and poliomyelitis • pneumococcal –> pneumonia • MMR or MMRV (varicella) –> measles, mumps, rubella and chickenpox • Human papillomavirus (HPV) –> cervical cancer

• may associate with serious side effects => preservatives or antibiotics in vaccines –> allergic reactions => adverse effects in vaccinees allergic to egg proteins as some vaccines are produced in chicken eggs => live attenuate viral vaccines –> infect foetus of vaccinated pregnant woman through placenta –> birth defects => oral polio virus vaccines may revert –> paralytic poliomyelitis => COVID-19 vaccines may lead to myocarditis in children