IMMUNOLOGY PUBLIC HEALTH HERD IMMUNITY ANTIGENIC VARIATION HERD
IMMUNOLOGY & PUBLIC HEALTH HERD IMMUNITY & ANTIGENIC VARIATION
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
HERD IMMUNITY THRESHOLD • - % of immune people above which a disease no longer persists • varies due to: • - pathogen’s virulence • - vaccine efficacy • - contact parameters
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
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
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
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