Antibody-Functionalized Inorganic NPs: Mimicking Nature for Targeted Diagnosis and Therapy - Bioinspired Approaches for Human-Centric Technologies - page 4

1.2 The Antibodies: Structure, Bioactivity, and Their
Employment
During the course of evolution, nature has developed immunoglobulins as part of
the defense machinery, the so-called humoral response, against foreign bodies,
cells, and molecules. Immunoglobulins, also known as antibodies, are complex
proteins produced and secreted by specialized cells of the immune system, the
lymphocyte B, in response to the interaction with an antigen. In mammals, there are
five different classes of immunoglobulins (IgM, IgG, IgA, IgD, and IgE), each of
them having a distinct role in the immune response and a different functional
location. The IgGs are the most abundant antibodies circulating in the serum, and
indeed they are the most studied and exploited in research and clinic. IgG anti-
bodies are glycoproteins with a mass of approximately 150 kDa and having a
typical Y-shaped conformation (Fig. 1.1 ). They consist of two identical functional
subunits, each of them made of two polypeptide chains, the heavy chain (50 kDa)
and the light chain (25 kDa). The two subunits are linked together through disulfide
bonds at the so-called hinge region. Also the two chains of each subunit are
connected by a S-S bond (Davies et al. 1990 ).
Both the heavy and the light chains consist of a variable region at the N-terminal
domain and of a constant region, at the carboxyl terminus. The constant region of
the heavy chain is approximately three times longer than that of the light chain,
while the variable regions of both chains have similar lengths. The two domains
contribute to the functionality of the antibody: the constant region of the heavy
chain is the effector domain which binds to the complement proteins and regulates
the immune response through the activation of the complement cascade, while the
variable domains are devoted to recognize and bind the antigen with high affinity.
Thanks to the high variability of this region, it has been estimated that about ten
billion different antibodies can be generated and thus an unlimited number of
antigens can be recognized (Amit et al. 1986 ).
Each antibody can bind two antigen molecules, which precisely fit in the pocket
formed by the variable regions of each subunit. The process of recognition of an
antigen is generally depicted as a “lock/key” system, but the binding strength of the
antibody-antigen complex may greatly vary. Antigen binding is mediated by
hydrophobic interactions, hydrogen bonds, and ionic interactions. Affinity and
specificity are the two parameters that describe respectively the strength of the
interaction and the ability of an antibody to discriminate between two different
antigens (Poljak 1991 ).
Starting from the 1970s and then in the following decades, new technologies
have been developed for the production and the engineering of large amounts of
antibodies (Maynard and Georgiou 2000 ). The advent of the hybridoma technology
first enabled the preparation of murine monoclonal antibodies, i.e., antibodies that
recognize a specific epitope of the antigen. The technique is called “hybridoma”
because it is based on the fusion of two cell populations. The first is a lymphocyte B
which produces a specific antibody and has been extracted from a mouse injected