Biomedical Engineering Reference
In-Depth Information
or skin tests, therefore, cannot be used to determine whether the patient will exhibit symp-
toms of IgE-mediated hypersensitivity upon exposure to an allergen. Assays for total
serum IgE, based on the same format as specific IgE immunoassays, are also available.
High serum levels of IgE may correlate to an atopic (i.e., allergic) state in the individual,
although the limitations of using total serum IgE as a diagnostic analyte include its age-
related concentration and the wide overlap in concentrations between atopic and
nonatopic individuals (6). The detection of total IgE, therefore, cannot result in a definite
clinical diagnosis, but rather would serve as a first-line test to guide clinicians in their deci-
sion-making process.
20.1.5
Affinity Biosensors
Affinity-based biosensors are devices incorporating a molecular recognition element
(MRE) such as an antibody, receptor protein, nucleic acid, molecular imprinted polymer
(MIP), or aptamer. The use of affinity-based biosensors has increased significantly over the
past decade (18). This is due to the progress made in signal transduction technologies, as
well as to the fact that MREs, especially antibodies, have become easier to isolate and
purify and hence have become more widely available. When an affinity biosensor includes
an antibody or antibody-related substance as the MRE, it is known as an immunosensor
(19). Affinity biosensors can be further classified based on the MRE incorporated into the
device and on the type of transducer used. The following is a brief overview of affinity
biosensors and their applications, in particular as applied to the detection of IgE.
20.1.5.1 Types of Affinity Biosensor
20.1.5.1.1 Immunosensors
Although immunosensors have been designed in a variety of different ways, incorporat-
ing different transduction technologies, they generally fit into one of three basic formats,
namely direct noncompetitive, competitive (direct or indirect), or sandwich (20) (see
Figure 20.4). In the direct noncompetitive format, the binding of the antigen to the anti-
body is directly detected based on the electrochemical or optical properties of the analyte
(Figure 20.4A). Surface plasmon resonance (SPR), discussed in Section 20.2, is a pertinent
example of direct optical detection of the binding event. Competitive assay formats are
generally used when the analyte is of a low molecular weight and the binding event can-
not be detected directly. In these formats, the signal is inversely related to the analyte con-
centration (18) (Figure 20.4B). The sandwich format is based on the enzyme-linked
immunosorbent assay (ELISA) principle, where the analyte tracer is conjugated to an
enzyme, such as horseradish peroxidase, and is detected after addition of a substrate
(Figure 20.4C). Most electrochemical transducers are based on this format, where the sec-
ondary antibody is usually conjugated to an enzyme capable of generating an electroac-
tive product after addition of a substrate (21).
Although electrochemical transducers are the most well characterized of the transducer
technologies (18), they do not lend themselves well to affinity-based sensors. This is
because most affinity-based reactions are not electrochemically active and, therefore, can-
not participate directly in redox reactions (21). In the case of amperometric immunosen-
sors, coupling of an immunoreaction to the transducer usually requires a labeled
immunoagent to facilitate the production of electroactive species, which in turn can be
detected electrochemically (21). The first report of an amperometric immunosensor was
from Aizawa et al. (22), who used catalase as a label to detect human chorionic
gonadotrophin. Major advantages of this type of electrochemical immunosensor include
