DETAILED DESCRIPTION A variety of devices can be used to contain or cloak a source of a therapeutic substance, often cells, which source provides the substance to a host or recipient subject.
Such devices include implantable devices, of both the diffusion and perfusion types, and extra corporeal devices, e.
g.
, those through which blood of the host or recipient is passed.
In such devices, host molecules can attack the source of the therapeutic substance and impair the function of the device.
Semipermiable components are used to inhibit the ability of host molecules to enter the device and attack the source of therapeutic substance.
The inventors have discovered that release, by the device, of components of the source of therapeutic substance, can stimulate a host response against the device and that capture of such components, prior to release or prior to becoming available to the immune system of the host, can improve the performance of the device, e.
g.
, by extending its useful lifetime.
The inventors have discovered that the inclusion in an implantable device of a capture agent, e.
g.
, an antibody directed against an antigen of the source of a therapeutic substance, which sequesters therapeutic source molecules, can improve the performance of the device, e.
g.
, by extending its useful lifetime.
Accordingly, the invention features, an implantable device which includes a source of a therapeutic substance, e.
g.
, an islet, and a capture agent, e.
g.
, an antibody which binds a component of the therapeutic substance, disposed within or on a semipermeable component.
The capture agents are preferably immobilized within the interior of the device or on its surface or are in a different compartment than the source of a therapeutic substance, or otherwise immobilized to keep it from contacting the source of a therapeutic substance.
The capture agent can, e.
g.
, coupled to a substrate, e.
g.
, an inert bead in a compartment of the microreactor, or can be coupled to the surface of the microreactor, or to the surface of a component of the microreactor
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Gene Therapy 
