Question 7 of 20 Fundamentals Concept
What do forward scatter and side scatter tell you about a cell?
Short answer
Forward scatter is light deflected at small angles along the laser path and roughly tracks cell size. Side scatter is light collected at about 90 degrees and tracks internal complexity, or granularity. Together they separate broad populations such as lymphocytes, monocytes and granulocytes, and flag debris and dying cells.
Why
Each cell crossing the laser scatters light. Forward scatter rises mostly with the cross-section of the cell, so bigger cells usually give more signal. Side scatter is reflected and refracted by internal structures such as granules and nuclear lobes, so granulocytes sit high on side scatter and lymphocytes low.
In blood, a plot of the two gives the classic three clusters before any antibody is added. Scatter is also the usual trigger, the signal a particle must exceed to be recorded at all, so a high forward scatter threshold can silently discard small cells or platelets. Dying cells typically shrink and lose forward scatter while gaining side scatter, so a scatter gate removes much debris.
The Cossarizza guidelines warn that it misses some dead cells, which is why a viability dye is the better tool. Scatter is not a true size measurement. Forward scatter also depends on refractive index, wavelength and the instrument's optics, so a fixed or permeabilized cell can look different from the same live cell.
For example, fixation often lowers forward scatter and changes side scatter. A scatter gate drawn on live cells may cut through the population after fixation.
A strong answer covers
- Forward scatter tracks size, side scatter tracks granularity
- Scatter separates leukocyte classes and flags debris
- Scatter is usually the trigger parameter and sets what is recorded
- Dead cells shift scatter but a viability dye is more reliable
- Fixation and optics change scatter, so it is not absolute size
Common mistakes
- Treating forward scatter as an exact measure of cell diameter
- Relying on a scatter gate alone to remove dead cells
- Setting the trigger threshold so high that small cells of interest are never recorded
The interviewer might ask next
How would fixation change the scatter gate you drew on live cells, and how would you adjust for it?