In this "Western Blot" assay, HIV viral proteins are separated according to size by electrophoresis (decreasing in size from top to bottom), then transferred to a membrane which is incubated with a patient's serum. If the serum contains antibodies which can bind to one or another viral protein, they will be rendered visible following incubation with an enzyme-labelled anti-immunoglobulin antibody, and a substrate which the enzyme will convert into a visible and insoluble product (blue, in this case).
The first three strips at the far left represent viral proteins incubated with three control sera, a "strong positive" (++), a "weak positive" (+), or a negative serum (-). A test serum is considered positive if it shows a visible band with two of the three proteins indicated by asterisks at the left. By this criterion, sera from patients #1, 4 and 7 are negative, while patients #2, 3, 5 and 6 are positive. For a diagram showing which bands represent which viral proteins, click here.