in some species. Providing that the antigen is large enough and that it does not
resemble proteins in the host animal then antibodies can be produced to a huge
number of substances which can be used in all branches of diagnostics and thera-
peutics. There are three types of antibodies that can be produced: these are polyclonal,
monoclonal and recombinant. Each of these antibody types has advantages but also
limitations and should be viewed as complementary to each other as each has specific
areas where they are particularly useful.
Polyclonal antibodies are produced in a number of animal species. Antibodies are
generated by immunising the host with the substance of interest usually three or
four times. Blood is collected on a number of occasions and the antibody fraction
purified from the blood serum. The exception to this is chicken polyclonal antibodies
which are harvested from eggs. Generally, larger animals are used since antibody is
harvested from the blood of the animal and bigger volumes can be obtained from
larger species. Historically, the first antibodies produced artificially for diagnostic
purposes were polyclonal. They are the cheapest of antibodies to produce and
have many uses in diagnostics. They have limited use in therapeutics, however, as
there are problems in that they themselves can be antigenic when injected into other
animals. There are exceptions to this and neutralising antibodies to snake venom
and prophylactic (reducing risk of infection) antiviral injections fall into this category.
Polyclonal antibodies are cheap to produce, robust but less specific than other
antibodies and will have variable qualities depending on the batch and specific
donor animal.
Monoclonal antibodies are secreted by mammalian cells grown in synthetic
medium in tissue culture. The cells that produce them are known as hybridomas and
are usually derived from donor mouse or rat lymphocytes. Human monoclonal
antibodies are also available but they are produced by different methodologies
to the rodent ones. The murine system was first described in 1976 when Kohler
and Milstein published their work. Monoclonal antibodies have radically altered
the possible uses for antibodies in both diagnostics and therapeutics. The basis
of the technology is the creation of the hybridoma by fusing antibody-secreting
B lymphocytes from a donor animal to a tumour cell line. B lymphocytes have a
limited lifespan in tissue culture but the hybridoma has immortality conferred by the
tumour parent and continues to produce antibody. Each hybridoma is derived from a
single tumour cell and a single lymphocyte and this has to be ensured bycloning. Cell
cloning is the process where single cells are grown into colonies, in isolation from
each other so that they can be assessed and the best chosen for future development.
Once cloned, the cell lines are reasonably stable and can be used to produce large
quantities of antibody which they secrete into the tissue culture medium that they are
grown in. The antibody they produce has the qualities that the parent lymphocyte had
and it is this uniqueness that makes monoclonal antibodies so useful. During immuni-
sation the B cells are presented with antigen fragments by macrophages and other
antigen-presenting cells and each cell then produces a specific antibody to the
fragment it has been presented with. The specific site that the antibody recognises is
known as an epitopewhich is approximately 15 amino acids in size. There are
thousands of potential epitopes on the antigen. The cell fusion process generates many268 Immunochemical techniques