CHAPTER I
INTRODUCTION
Immunity in the vertebrates may be defined as a response of an
animal to a foreign substance (an antigen or immunogen) introduced into
its body. The response is specific in that it is directed only to the
antigen introduced and is characteristically more pronounced and occurs
sooner if the same antigen is reintroduced at a later time (43) .
Immune responses in birds and mammals may be cellular (specifically
reactive cells) or humoral (antibody mediated) (43,54,93). Characteris
tic cellular responses are delayed type hypersensitivity reactions, graft
rejection and graft-versus-host reactions. Specific cellular responses
are transferable by lymphocytes. Humoral responses are characterized
by the production of antibody directed to an antigen and the resulting
immunity is transferable by serum.
The lymphocyte has been demonstrated to be the principal cell type
involved in the immune responses of birds and mammals. Although lympho
cytes are morphologically identical, two major subpopulations have been
identified based on ontological origin and functional analysis (43,54,
93). One subpopulation, the T-(thymus derived) lymphocyte, is the func
tional cell in cellular mediated responses. The other subpopulation,
the B-(bursa derived in birds or bursal equivalent in mammals) lympho
cyte, is the functional cell in producing antibodies in humoral responses.
T- and B-lymphocytes have been further characterized on the basis of cell
surface determinants and in vitro responses to mitogens, antigens and
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