Now, researchers at the University of Pennsylvania School of Engineering and Applied Science and its Institute for Translational Medicine and Therapeutics have figured out a way to provide a "passport" for such therapeutic devices to sneak in past the body's security system.
In 2008, the group led by Dennis Discher, professor at Pennsylvania University, showed that the human protein CD47 is found on almost all mammalian cell membranes.
Like a border guard inspecting a passport, it tells the macrophage (white blood cells) that the cell or an object isn't an invader and should be allowed to proceed, the journal Science reports.
"There may be other molecules that help quell the macrophage response," Discher said. "But human CD47 is clearly one that says, 'Don't eat me'."
"From your body's perspective... an arrowhead a thousand years ago and a pacemaker today are treated the same -- as a foreign invader," said Pia Rodriguez, a graduate student working under Discher, according to a Penn statement.
"We'd really like things like pacemakers, sutures and drug-delivery vehicles to not cause an inflammatory response from the innate immune system," Rodriguex added.
The innate immune system attacks foreign bodies in a general way. Unlike the learned response of the adaptive immune system, which includes the targeted antibodies that are formed after a vaccination, the innate immune system tries to destroy everything it doesn't recognize as being part of the body.
Drug-delivery nanoparticles naturally trigger this response, so researchers' earlier attempts to circumvent it involved coating the particles with polymer "brushes".