Discrimination of granulocyte subtypes from light scattering: theoretical analysis using a granulated sphere model

We perform extensive simulations of light scattering by a granulated sphere in the size and refractive index range of human granulated leucocytes using the discrete dipole approximation. We calculate total and depolarized side scattering signals as a function of the size and refractive indices of cell and granules, and the granule volume fraction. Using typical parameters derived from the literature data on granulocyte morphology, we show that differences between experimentally measured signals of two granulocyte subtypes can be explained solely by the difference in their granule sizes. Moreover, the calculated depolarization ratio quantitatively agrees with experimental results. We also use the Rayleigh-Debye-Gans approximation and its second order extension to derive analytical expressions for side scattering signals. These expressions qualitatively describe the scaling of signals with varying model parameters obtained by rigorous simulations, and even lead to quantitative agreement in some cases. Finally, we show and discuss the dependence of extinction efficiency and asymmetry parameter on size and volume fraction of granules.