  The real, interacting elementary particle always consits of a 'bare' particle
and a cloud of virtual particles mediating a self-interaction and/or the bond
inside a composite object. In this letter we discuss the question of spin
content of the virtual cloud in two different cases: electron and quark.
Further, the quark spin is discussed in the context of proton spin, which is
generated by the interplay of quarks and virtual gluons. We present a general
constraint on the gluon contribution and make a comparison with the
experimental data.
