INTRODUCTION
This chapter is not a comprehensive review
of immunology but rather a condensed
version of those aspects of immunology
that have particular relevance to clinical
immunology. Refer to the Bibliography for
a more extensive discussion of the role of
each component.
It is generally believed that the immune
system evolved as the host’s defense against
infectious agents, and it is well known that
patients with defi ciencies in the immune
system generally succumb to these infec-
tious diseases. However, as we shall see, it
may well play a larger role in the elimina-
tion of other foreign substances, including
tumor antigens or cells and antibodies that
attack self.
An immune response may be conve-
niently divided into two parts: (1) a specifi c
response to a given antigen and (2) a more
nonspecifi c augmentation to that response.
An important feature of the specific
response is that there is a quicker response
to the antigen during a second exposure to
that antigen. It is the memory of the initial
response that provides the booster effect.
For convenience, the specifi c immune
response may be divided into two parts:
(1) the humoral response and (2) the cellu-
lar response to a given antigen. As we shall
see, however, both responses are medi-
ated through the lymphocyte. Humoral
responses are antibodies produced in
response to a given antigen, and these anti-
bodies are proteins, have similar structures,
and can be divided into various classes of
immunoglobulins. Cellular responses are
established by cells and can only be trans-
ferred by cells. (See the Bibliography for
the extraordinary beginnings of the con-
cept of a cellular arm of the immune sys-
tem.) Up to the 1940s the general dogma
held that only antibodies were involved in
the immune response. Dr. Merrill Chase,
who began his experiments in a labora-
tory devoted primarily to the humoral
response, clearly showed in a series of ele-
gant experiments that immunity was not
just humoral but that a cellular response
by the lymphocytes could also produce
immunity. Some of the best examples of
the power of cellular immunity may be
found in the many experiments in which
transfer of cells can induce autoimmune
disease in animals and humans as well as
rejection of an organ graft in both animals
and humans by cells.
The separation of human and cellular
immunity was further advanced by the
study of immunodefi cient humans and
animals. For example, thymectomized or
congenitally athymic animals as well as
humans cannot carry out graft rejection,
yet they are capable of producing some
antibody responses. The reverse is also
true. Children (and animals) who have an
immune defi cit in the humoral response
do not make antibodies but can reject