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490 The Toxicology of Fishes

macrophages, granulocytes, and natural-killer-like cells) and noncellular (i.e., lysozyme, complement,
and acute-phase proteins) aspects of nonspecific immunity, as well as specific (adaptive) immune
responses that rely upon acquired immunity (i.e., cell- and humoral-mediated immunity), are discussed.

Immune Cells
Many morphological and functional similarities exist between mammalian and teleostean leukocytes
(Rowley et al., 1988). Lymphocytes, macrophages (MØs)/monocytes, granulocytes (neutrophilic, eosi-
nophilic, and basophilic), and nonspecific cytotoxic cells (NCCs) have all been described in a variety
of teleost models; however, species-specific differences in occurrence, morphology, and function are
common. These differences make generalizations about fish leukocytes difficult. Cells involved in the
nonspecific cellular response include MØs, granulocytes, and NCCs. All of these cell types are vitally
important for inflammation and probably represent the most primitive group of immune-effector cells.
Although excellent descriptions of MØs (Secombes, 1996; Secombes and Fletcher, 1992), granulocytes
(Ainsworth, 1992; Secombes, 1994; Suzuki and Iida, 1992), and NCCs (Evans and Jaso-Friedmann,
1992; Secombes, 1996) can be found elsewhere, a brief review of cell function is provided here (see
discussion on nonspecific immune defenses). It appears that jawed vertebrates (gnathostomes) are the
most phylogenetically primitive organisms to possess lymphocytes. Both B- and T-lymphocytes have
been described in teleosts due primarily to functional and structural similarities shared by their mam-
malian counterparts. The presence of antibody in bony fish led early investigators to believe that fish
possessed antibody-producing B-lymphocytes. The expression of surface immunoglobulin (sIg) on fish
lymphocytes has since been described in a number of species, including channel catfish (Ictalurus
punctatus) (Lobb and Clem, 1982), carp (Cyprinus carpio) (Koumans-van Diepen et al., 1994), and sea
bass (Dicentrarchus labrax) (Romestand et al., 1995). Teleost T-lymphocytes are generally recognized
as surface immunoglobulin-negative lymphocytic cells, and T-lymphocyte-specific monoclonal antibod-
ies have been produced for some species (Partula, 1999; Scapigliati et al., 1999). The structure and
known functions of fish lymphocytes are described in detail later in this section.

Lymphoid Organs and Tissues
The lymphoid tissues and organs of teleost fish have previously been reviewed (Press and Evensen,
1999; Zapata et al., 1996). Unlike mammals, fish lack hematopoietic bone marrow and identifiable lymph
nodes. The primary site of hematopoiesis in teleosts is the pronephros or anterior portion of the kidney
(commonly referred to as the head or cranial kidney). Teleosts also possess a thymus and spleen (although
splenic germinal centers are absent) along with mucosal lymphoid tissues. The morphology of fish
lymphoid organs is highly species specific, although some generalizations about structure and function
can be made.

Thymus
In most teleosts, the thymus is a paired organ located dorsolaterally above the opercular cavities
(Chilmonczyk, 1992). The thymus has been found anywhere from just beneath the opercular epithelium,
as in rainbow trout (Oncorhynchus mykiss) (Chilmonczyk, 1985) to a more internalized location as
described in angler fish (Fange and Pulsford, 1985). Histologically, the thymus is poorly differentiated
into cortical and medullary regions, with as many as six different zones of varying cell densities (Zapata
et al., 1996). Figure 11.1 demonstrates the thymus from Japanese medaka (Oryzias latipes).
As in higher vertebrates, the fish thymus is believed to be the first lymphoid tissue to become populated
by lymphocytes during development. The ontogenesis of the thymus gland has been described for many
fish species, including rainbow trout (Oncorhynchus mykiss) (Chilmonczyk, 1985; Grace and Manning,
1980), carp (Cyprinus carpio) (Romano et al., 1999a; van Loon et al., 1982), zebrafish (Danio rerio)
(Willett et al., 1997, 1999), sea bass (Dicentrarchus labrax) (Abelli et al., 1996), catfish (Ictalurus
punctatus) (Petrie-Hanson and Ainsworth, 2001), and tilapia (Oreochromis niloticus) (Fischelson, 1995).
Involution of the teleost thymus has also been described following exposure to various environmental