3.2.4: Cell recognition and the immune system (Viruses (HIV) and ELISA testing)

HIV (human immunodeficiency virus) is a virus which causes AIDS (acquired immune deficiency syndrome) by attacking helper T cells and interfering with their function, reducing the amount of T helper cells in the blood. Without a sufficient number of T helper cells the immune system cannot stimulate enough B cells to produce enough antibodies to combat pathogens/enough cytotoxic T cells to kill infected cells. This means the body is unable to produce enough of an immune response, and this also makes the patient more susceptible to cancers and other infections. It is these infections/diseases that cause death, not the AIDS itself.

HIV belongs to a group of viruses known as retroviruses. The structure:

• Lipid envelope
• Attachment proteins
• Capsid
• Two single strands of RNA
• Enzymes (including reverse transcriptase which catalyses the production of DNA and RNA)

HIV cannot replicate itself as it is a virus. It replicates by...

• HIV enters the bloodstream and circulates around the body
• A protein on HIV binds to a protein on helper T cells
• The capsid fuses with the cell-surface membrane of the T helper cell
• RNA and enzymes enter the T helper cell and the reverse transcriptase converts the viral RNA into DNA (so the host cell can read it).
• This DNA is inserted into the host cells DNA in the nucleus. This DNA creates mRNA which contains instructions for creating new viral proteins and RNA for the new HIVs
• mRNA passes out of the nucleus and it is read and proteins are made
• HIV particles break away from the T helper cell and take a piece of it's cell-surface membrane with it which forms the new lipid envelope.

We also need to know about ELISA testing, not sure where to put it so here it is:

ELISA (enzyme linked immunosorbant assay) uses antibodies to quantify the amount of a protein in a sample. It is useful in both allergen and drug tests. Here is an example of how it works when testing for antigens:

• Apply a sample to the surface of a slide
• Wash the surface to remove any unattached antigens
• Add the antibody that is specific to the antigen, leave for a little bit so the two can bind together
• Wash the surface (again) to remove excess/unbound antibodies
• Add a second antibody that has an enzyme attached to it. This will bind with the first antibody
• Add the colourless substrate of the enzyme
• When the enzyme acts on the substrate, colour is produced
• The amount of antigen present is proportional to the intensity of the colour of the final solution.

Unfortunately, we cannot use antibiotics against viruses. This is because one way in which viruses work is by inhibiting enzymes required for the synthesis of peptide cross-bridges in cell walls. This means that, in bacteria, they can no longer withstand osmotic pressure as their cell wall is very weak and they burst. However, viruses do not have a cell wall. They also do not have any metabolic mechanisms that the antibiotic might be able to disrupt.