Yes, it is perfectly healthy to swallow your phlegm.
Spit the phlegm out; don't swallow it. Among the many
components that make up phlegm are substances like immunoglobulins
and glycoproteins that help your body fight off an infection. When
these substances get killed (together with the bacteria and virus
they were fighting), they become part of what makes up phlegm. When
phlegm gets coughed out, it means that they have served their
purpose - remnants and refuse of the battle, so to speak - and are
no longer necessary in the proper functioning of the body. They
have to be expelled. Therefore, spit them out and don't swallow
them back. Aside from the fact that swallowing phlegm is gross and
unsanitary, it sometimes gets reintroduced to your pulmonary
system, worsening your situation.
Nope! Acidity of the stomach is sure to destroy the bacteria. On
the contrary, spitting the phlegm out is gross, even though into a
napkin. As to the substances like immunoglobulins and
glycoproteins, they are played out yes, however, it is very well
known that they cannot be absorbed from the intestines as whole.
Instead, the aminoacids and other monomers that constitute them are
absorbed and re-used. Consider that we intake the proteins of
someone else (an animal's). We use them after digesting
It may be healthy, but swallowing too much phlegm may cause it to clog up in your throat and turn into a nasty cough.