Biomedical Engineering Reference
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produced without the involvement of T cells. Equally important is
that the B cells expressing a receptor with high affinity for antigen
are expanded and that the maturation also involves the formation of
so-called memory B cells. Thus, the recognition of antigen selects B
cell clones that express a receptor appropriate for such recognition
and hence is able to produce secreted antibody also recognizing
that antigen. Memory B cells enable a quick formation of antibodies
to microbial antigens when these are encountered a second time,
which eff ectively serves to limit the spread of an infectious agent in
the body. The observation that cells of the immune system support
recall response is a hallmark of adaptive immunity. As a consequence,
the function of the immune system is dependent on past exposures
to antigens, which obviously introduces a significant variability in
the immune responses among any group of individuals. Although
antibodies are selected on the basis of their ability to form complexes
with an antigen, this does not imply that such antibodies may only
bind to this antigen. Cross reactivity often permits antibodies to bind
substances structurally similar to their cognate antigen [7-10]. While
to some extent it is possible to predict the antigenicity of proteins
with a known structure [11], the considerable variation between
healthy individuals in the reactivity of antibodies [12] makes it
essentially impossible to predict if antibodies to antigens, e.g., in
formulations involving nanomaterials, may have been preformed.
Fortunately, several tests are now possible to broadly investigate
if antibody reactivity would allow for the binding of Ig to chosen
materials [12].
Secreted antibody to non-proteinous antigens such as lipids
or carbohydrates can also be produced, but this process involves
T cells only in a limited way and the response is usually weaker
than that found for protein antigens. This process depends on the
ability macromolecules with a repeated structural motif to cluster or
cross-link several B cell receptors in the cell membrane. In addition,
immunoglobulins are also produced in a way that appears to not
be driven by antigen recognition or T cell involvement. Based on
studies in mice, it appears that these so-called natural antibodies are
produced by a specialized subset of B lymphocytes [13]. Insight into
the biology of natural antibodies in man is lacking. The exact mode
of antigen recognition by these antibodies remains enigmatic, but
their binding profile suggests promiscuous interactions with many
protein and non-protein ligands. In the context of nanotechnology
 
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