Biomedical Engineering Reference
In-Depth Information
High ai nity interactions can be considered almost complete in a sub-
stantially shorter time than low ai nity interactions even when in theory
the time to reach the actual equilibrium is independent from the ai n-
ity. High ai nity complexes are said to be much more stable and indepen-
dent. Ai nity constants K
A
range from 10
5
to 10
12
M
-1
.h e tailored binding
sites of the epitopes are responsible for achieving high ai nities. 1,000 fold
decreased ai nity is brought about by the loss of one hydrogen bond in
an interaction. Whereas the ai nity constant for a monoclonal antibody
can be determined and known that, K
A
for polyclonal serum are more dif-
i cult to be determined. Monoclonal antibodies consist of identical immu-
noglobulin molecules whereas Polyclonal antibodies represent a mixture
of antibodies, specii c for the antigen. h e ai nity of their components
determines the specii cations of immune-sensors are determined to a large
extent. High ai nity results in sensitive sensors, but very high ai nities
might result in virtually irreversible sensors.
7.8.1.4 Avidity
h e term avidity is dei ned as the measure of the overall stability of the
antibody antigen complex [140]. For immunochemical reactions avidity
is of more evidential importance than ai nity, as it refers to the intrinsic
ai nity of the paratrope such as for the epitope, the valency of the anti-
body and the geometric arrangement of the interacting compounds in the
nature. High avidity is reached when all the paratropes has an ability to
bind epitopes. For the IgM there can be ten epitopes known. Avidity is
also signii cantly increased by multivalent systems, where many antibodies
eventually bind to dif erent epitopes of the same antigen and also crosslink
the antigens involved. It is easily achieved by using polyclonal antibodies
representing mixture of antibodies and large antigens with multiple epit-
opes. Even cross-linking (higher avidity) has been highly achieved by add-
ing anti-IgG immunoglobulin, beads of protein A or protein G. Avidity is
increased by relatively dense antigen layers by allowing the bivalent bind-
ing and also increases the overall stability of the antibody antigen complex
for any surface bound antigen.
7.8.1.5
Antibody Production-Polyclonal antibody
An immunogen i.e. analyte or analyte conjugate is injected into a host
animal for the production of polyclonal antibodies. At er immunisation,
the host species, e.g. mouse, goat rabbit or sheep reacts specii cally with
a primary immune response and mainly because of it IgM is produced.
In the following days and weeks, the host animal is injected again (boost
