"Fold Change" is one way of comparing a signaling response of a population of cells to the basal, unstimulated state.  

On Cytobank, the default is to calculate Fold Change using a linear comparison of median fluorescence intensity (MFI) values.  For more information, you can read about the different Statistics available on Cytobank, which include MFI, mean, geometic mean, percentage in a gate, and percentile.  Signaling responses can be calculated either as a linear ratio or using the scale of the display (e.g. log10 fold change).  On Cytobank, the term "Fold Change" by itself refers to the linear ratio.

To calculate the Fold Change of a stimulated sample, relative to an unstimulated sample, the equation is most often:

MFI stimulated / MFI unstimulated

For example:

MFI unstimulated: p-STAT5 MFI of unstimulated, resting CD3+ T cells: 122

MFI stimulated:  p-STAT5 MFI of CD3+ T cells, 15 minutes following IL-2 stimulation: 4,252

Fold change = MFI stim / MFI unstim = 4,252 / 122 = 34.9

You could state, "At 15 minutes following IL-2 stimulation, a 34.9 fold increase in the MFI of p-STAT5 was observed in the CD3+ T cell population."

Fold Change can be calculated between any two samples, so we might generalize the equation as:

MFI Sample B / MFI Sample A

In the case where the MFI of Sample B is less than the MFI of Sample A, we calculate the fold decrease instead of calculating a fractional fold increase.  In this case, the following equation can be used:

0 - (MFI Sample A / MFI Sample B)

For example:

MFI Sample A:  p-STAT5 MFI of CD3+ T cells, 15 minutes following IL-2 stimulation: 4,252

MFI Sample B:  p-STAT5 MFI of CD3+ T cells, 3 hours following IL-2 stimulation: 146

Fold change = 0 - (MFI Sample A / MFI Sample B) = 0 - (4,252 / 146) = -29.1

You could state, "Between 15 minutes and 3 hours following IL-2 stimulation, a 29.1 fold decrease in the MFI of p-STAT5 was observed in the CD3+ T cell population.  By 3 hours, the signal had essentially returned to the basal state and was only 1.2 fold above the level observed in resting cells."