1. Immunology & Public Health
Herd immunity & antigenic variation

2. Herd immunity
If most of the population have immunity against an infection, it is less likely for the infection to spread
Non-immune individuals are protected by the “herd”
Mass vaccinations have helped reduce disease spread
E.g. TB, polio and smallpox

3. Herd immunity threshold
- % of immune people above which a disease no longer persists
varies due to:
- pathogen’s virulence
- vaccine efficacy
- contact parameters

4. Public health medicine
Public health policies help to create herd immunity
In developing countries, poverty and malnourishment limit public health policies
- especially mass vaccination programs
In developed countries, adverse publicity of vaccinations can affect herd immunity thresholds

5. Evasion of specific immune response
Change in genotype:
- mutations and genetic recombination
Antigenic variation:
- antigens vary from original strain
Influenza virus:
- new antigens can’t be detected by memory cells
Trypanosoma protozoa
- causes “sleeping sickness” (trypansomiasis)
- surrounded by glycoprotein
- hundreds of genes code for variants of the glycoprotein
Malaria
- high level of genetic variability
- infect red blood cells with a surface protein
- infected blood cells adhere to blood vessels (can’t be removed and destroyed)
- parasite can switch genes for this protein

6. Direct attack on the immune system
Tuberculosis (TB):
Survives inside phagocytes
- prevents lysosomes fusing with phagocytic vesicle
- also waxy cell wall prevents attack by lysosomal enzymes
- survives and multiplies inside phagocytes
- avoids immune detection
AIDS & HIV:
HIV attacks helper T cells
- attaches to specific cell receptors
- virus membrane fuses with cell membrane,
HIV is a retrovirus
- uses RNA, and reverse transcriptase to reverse normal transcription
- viral DNA incorporates into cell DNA
- allows new viruses to be produced
- viruses released by “budding”
- antibodies are ineffective – HIV is hidden
- fewer helper T cells survive
- individual more susceptible to other infections
E.g. pneumonia, cancer