  Realtime analyses are necessary to identify the source of high energy
neutrinos. As an observatory with a 4$\pi$ steradian field of view and
near-100% duty cycle, the IceCube Neutrino Observatory is a unique facility for
investigating transients. In 2016, IceCube established a pipeline that uses
low-latency data to rapidly respond to astrophysical events that were of
interest to the multi-messenger observational community. Here, we describe this
pipeline and summarize the results from all of the analyses performed since
2016. We focus not only on those analyses which were performed in response to
transients identified using other messengers such as photons and gravitational
waves, but also on how this pipeline can be used to constrain populations of
astrophysical neutrino transients by following up high-energy neutrino alerts.
