Memory cells

Question 1

Memory cells - once seen, never forgotten

Answer 1

 Vaccines produce their protective effect by inducing active immunity and providing immunological memory
 Immunological memory enables the immune system to recognise and respond rapidly to exposure to natural infection at a later date – prevents or modifies the disease
 The key to an effective vaccine is the ability for it to trigger proliferation of naïve T-cells
 Success depends on whether T-helper1 cells and CTL responses are induced against
intracellular organisms –viruses
 T-helper2 antibody responses are induced against extracellular organisms – bacteria and
parasites, or their toxins

Question 2

Role of adjuvants

Answer 2

 Many soluble proteins are poorly immunogenic when used on their own as a vaccine
 Adjuvants are compounds that enhance immunogenicity of the protein antigens
 They do this in 2 ways
1. By converting soluble proteins into particles – can use alum (adsorbs proteins), mineral oils (emulsifies proteins) or Quil A detergent (forms colloids with proteins)
2. Effect is enhanced by including bacterial components – activates APCs and/or induces cytokine production and enhances inflammatory responses
 Latter can be toxic – many not licensed for use in humans
 Many APC receptors are now mapped and ligands are identified – provides co-
stimulatory signals to activate T-cells, along with MHC + antigen peptide

Question 3

Principles of safe and effective vaccination

Answer 3

 Search for attenuated organisms with reduced pathogenicity to stimulate protective
immunity
 Develop inactivated (killed) organisms – won’t cause lethal systemic infection in the immunosuppressed
 Develop purified components of whole organisms containing key antigens that stimulate protective immunity

Question 4

Requirements for an effective vaccine

Answer 4

 Depends on the organism
 Intracellular organisms usually need CTL and antibodies
 Extracellular organisms usually need neutralising antibodies

Question 5

It provides defence at point of entry

Answer 5

 Stimulation of mucosal immunity may be required at specific mucosa (gut, respiratory epithelia, gut epithelia

Question 6

Pre-existing antibody may be required

Answer 6

 Pre-existing antibody protects against diphtheria/tetanus exotoxins – forms
antigen-antibody complexes which are phagocytosed
 Polio and HIV enter cells shortly after infection – antibodies must block ligand-cell
receptor interaction

Question 7

Protection optimised when specific epitopes are recognised

Answer 7

 Immune responses directed at multiple epitopes
 Not all of these generate protective antibodies or CTLs
 Some may even generate suppressor T-cells
 Correct epitopes must be targeted

Question 8

It must be safe

Answer 8

Must have very low toxicity

Question 9

Must protect most vaccines

Answer 9

 Rapid generation of herd immunity

 Reservoir of susceptibles falls – transmission drops

Question 10

Must generate long-lived immunity

Answer 10

 Repeated booster immunisations often impracticable

 Cheaper and improves population health

Question 11

It must be cheap

Answer 11

 Will be administered to large populations
 Very cost-effective healthcare
 But benefit reduced if cost per dose rises