5
existence of the so-called hapten-carrier effect (46,106,121). Since
this "helper effect" is considered to result from T-B cell collaboration
in mammals it appears that fish may also have this coordinated function
in their immune response system.
Since fish are ectothermic animals it is not surprising that numer
ous reports of temperature influences on immune responses have appeared.
The classic studies of Bisset (10), Cushing (40) and Hildemann and Cooper
(61) demonstrated that temperature can have a profound role in these
responses. The more recent studies of Avtalion have served as a basis
for beginning to understand the mechanism of these effects (7). He has
shown that humoral responses in the carp are a two-step process: 1) a
temperature-sensitive step requiring relatively high temperatures for
antigen recognition and 2) a temperature-insensitive step which results
in antibody production. Furthermore, Avtalion suggests that there are
at least three cell types involved; 1) X cells (T-like) which are sensi
tive to low temperatures and are involved in priming and tolerance and
2) Y and Z cells (B-like) which are involved in memory and antibody for
mation respectively. It must be pointed out however that direct proof
for the existence of multiple types of immunocompetent cells in fish is
lacking.
More recently Etlinger et^ al. (46) presented evidence that rainbow
trout have two lymphoid cell types. This evidence is based on responses
of leukocytes isolated from various lymphoid organs to the mammalian T
and B cell mitogens. Thymocytes responded only to Con A (a T-cell mito
gen in mice and man) and anterior kidney leukocytes responded only to
I.PS or PPD (B-cell mitogens). The unique pattern of tissue localization
of cells responsive to mammalian T- and B-lymphocyte mitogens was taken
as evidence for lymphocyte heterogeneity in rainbow trout.