How Vaccines Work

Question 1

Immunity

Answer 1

 The ability of the human body to protect
itself from infectious disease
 The defence mechanisms of the body are
complex and include:
 Innate mechanisms – non-specific
and non-adaptive
Acquired systems – specific and adaptive

Question 2

Innate/non-specific immunity

Answer 2

 Present from birth
 Includes:
 Physical barriers e.g. intact skin and mucous membranes
 Chemical barriers e.g. gastric acid, digestive enzymes and bacteriostatic fatty
acids of the skin
 Phagocytic cells and the complement system

Question 3

Enhances the body’s immune response to an antigen

-Adaptive/acquired immunity

Answer 3

 Acquired immunity is usually specific to a single organism or to a group of closely related organisms – share common antigens
 2 basic mechanisms for acquired immunity – active and passive
 Active acquired immunity

Question 4

How(Active acquired immunity):

Answer 4

 Produced by an individual’sown immune system – usually
long-lasting
 Involves cellular responses (cell-mediated), humoral
responses (antibody-
mediated) or a combination acting against one or more antigens on the infecting organism
 Can be acquired by natural disease or by vaccination
 Vaccines provide immunity similar to that provided by the natural infection, but without the risk from the disease or its complications

Question 5

The Adaptive immune system -CD4 LINEAGE

Answer 5

 APCs see antigen
 They chop it up and present it
as peptides with MHC class II to
naïve CD4 T-helper cells
 They then become effector T-
cells of 4 basic types

Question 6

Antibody mediated HUMORAL IMMUNITY

Answer 6

Antibody provides immunity against infection by:
1. Neutralising bacterial toxins, bacterial adhesins and viruses by blocking and binding
to cell-surface receptors = neutralising antibodies
2. Targeting CTLs to infected cells – antibody-dependent cellular cytotoxicity (ADCC)
3. Coating/opsonising pathogens and targeting them for phagocytosis
4. Antibody-antigen complexes activate classical complement cascade which leads to
destruction of pathogens by phagocytosis or bacterial membrane attack

Question 7

Roles of antibody subtypes in protection

Answer 7

 IgM: major role in complement activation – limited
role in neutralisation and induced by vaccination
(primary response) – blood
 IgG: systemic (blood, tissues, lymph) – all major
roles (neutralisation, ADCC (antibody-dependant cellular cytotoxicity), opsonisation, complement activation) – induced by vaccination
 IgA: principle isotype in secretions at mucosa (gut, respiratory tract, genital) – less potent at opsonising,a weak activator of complement but strong viral neutraliser – can be induced by
vaccination (but needs adjuvants)
 IgE – Helminths
 IgD – no role

Question 8

Passive antibody immunisation

Answer 8

 Passive immunity = protection provided by the transfer of antibodies from immune
individuals, most commonly across the placenta (IgG) and from breast milk (secretory
IgA)
 Protection from cross-placental antibodies are more effective against some infections
(tetanus and measles) than for others (polio and whooping cough)
 This protection is temporary – last for a few weeks or months, as neonates are very
vulnerable to disease
 Mother should have an up-to-date vaccination profile, in particular MMR – to protect
their child (rubella can damage the foetus irreversibly)
 As healthy immune system is dependent on nutritional status, mothers should also
eat well

Question 9

Cell-mediated immunity

Answer 9

 Cell-mediated immunity is provided by effector
lymphocytes (T-cells) of T-helper 1 type (includes
CD8 CTLs)
 2 principal classes – each has specific function
1. CD8 T-cells/cytotoxic/killer T-lymphocytes – recognise and destroy infected cells
2. CD4 T-helper cells – activate phagocytic macrophages to destroy engulfed bacteria and activate B-cells to produce IgG1 and IgG2 subtypes – strongly opsonising