  Motivated by the reported peculiar dynamics of a red blood cell in shear
flow, we develop an analytical theory for the motion of a nearly--spherical
fluid particle enclosed by a visco--elastic incompressible interface in linear
flows. The analysis explains the effect of particle deformability on the
transition from tumbling to swinging as the shear rate increases. Near the
transition, intermittent behavior is predicted only if the particle has a fixed
shape; the intermittency disappears for a deformable particle. Comparison with
available phenomenological models based on the fixed shape assumption
highlights their physical foundations and limitations.
