Vaccination, HIV and antibioticsAQA A-Level Biology: Revision notes

A vaccine contains antigens from a pathogen, for example dead or weakened pathogens, or their antigens only. It stimulates a primary immune response, producing memory cells. If the person meets the live pathogen later, the secondary response is rapid and large, so the pathogen is destroyed before symptoms appear.

Herd immunity: if a high enough proportion of a population is vaccinated, the pathogen cannot spread easily because there are few susceptible hosts. This also protects people who cannot be vaccinated, such as babies and those with weakened immune systems.

Active immunity is produced when the person makes their own antibodies, after infection or vaccination. It involves memory cells, so it is long-lasting, but takes time to develop.

Passive immunity is produced by antibodies from another source, for example across the placenta, in breast milk or by injection. It acts immediately but is short-term, because the antibodies are broken down and there are no memory cells.

The human immunodeficiency virus (HIV) has:

• a core of two molecules of RNA and the enzyme reverse transcriptase
• a protein capsid
• a lipid envelope from the host cell, carrying attachment proteins

HIV infects helper T cells:

1. The attachment protein binds to a receptor on the helper T cell, and the capsid enters the cell.
2. The RNA is released, and reverse transcriptase makes DNA from it.
3. The DNA is inserted into the host cell's DNA.
4. The host cell makes viral proteins and RNA, which are assembled into new virus particles that leave the cell.

HIV destroys helper T cells. As their number falls, fewer cytotoxic T cells and B cells are stimulated, so fewer infected cells are killed and fewer antibodies are made. The immune system is weakened, and the patient suffers opportunistic infections (such as TB and pneumonia) and some cancers. This stage is AIDS.

Antibiotics kill bacteria or stop them growing by targeting structures and processes that bacteria have, such as the cell wall or bacterial enzymes. Viruses have no cell wall, no ribosomes and no metabolism of their own. They replicate inside host cells using the host's machinery, so antibiotics are ineffective against viruses.

Ethical issues with vaccines include testing on animals and humans (informed consent, a placebo group left unprotected), possible side effects, whether vaccination should be compulsory or a personal choice, cost, and fair access.

To evaluate evidence from a vaccine trial, look at:

• the sample size and whether groups were randomly allocated
• the use of a placebo or control group
• the numbers of cases in each group, calculated as percentages or per 1000
• the length of the trial, since protection must last
• who was included, for example immunosuppressed people