We derive the dispersion forces between objects in the presence of a non-thermal radiation field. We apply the formalism to a model system representing two human blood cells in blood. By focusing the same radiation density, as in room-temperature thermal radiation, in the microwave region we find a huge enhancement of the attractive force. Related effects are predicted to occur also in other types of biological tissue. The quantitative results should not be taken at face value, since the model is crude. The effects are so large though, that further investigation is motivated. The origin of the effects lies in the variation of water and/or ion content. In the microwave region of the spectrum both the dipolar contributions from the water-molecules, and the mobile-ion contributions are important parts of the dielectric function, and cause the enhancement.