Invisible Plasma Content in Blazars? The Case of Markarian 421

Invisible plasma content in blazar jets such as protons and/or thermal electron-positron ($e^{\pm}$) pairs is explored through combined arguments of dynamical and radiative processes. By comparing physical quantities required by the internal shock model with those obtained through the observed radio-to-gamma-ray spectra for Mrk 421, we find the existence of a copious amount of invisible plasma in the jet. We speculate that the blazar sequence could arise from variations of total amount and/or blending ratio of $e^{\pm}$ pair and electron-proton plasma.