The immune response depends on a complex, interrelated system of organs, tissues, cells and soluble protein factors. Ultimately, however, the function of the immune system is very simple: to protect the living organism from foreign material. Assays of immune function span a wide range of scale, from the study of isolated cells in vitro to disease challenges of intact organisms. Each of these assays has its own set of strengths and weaknesses. The selection of any particular immunotoxicity assay depends both on the specific questions to be answered and on the nature of the experimental program.

General assessments include endpoints such as histopathological examination of immune tissues, hematocrits, leukocrits, and measurement of serum levels of various soluble factors. Functional assessments involve direct assessments of the capacity of immune cells to carry out their respective functions. Functional assessments include phagocyte assays, reactive oxygen production, cytotoxicity tests, lymphocyte proliferation assays, plaque-forming cell assays, and antibody measurement. Host resistance assays are considered by many to be the ultimate tests of immune function. In these studies, living organisms are exposed to some stressor, then to some infectious agent or parasite. Resistance to infection is the endpoint of interest in these assays. Interpretation of the results of immunotoxicity assays is complicated by a high degree of inter- and intra-species variability in response, as well as the fact that immune response is influenced by season, sex, age, hormonal status, stress levels, breeding condition, diet, and temperature.

If the need arises for an aquatic biology research activity to provide information on the potential effects of industry effluents on fish immune function, it should be noted that a considerable amount of data on general assessments already exists in the records of previous research. Assessments of phagocytic activity could also provide valuable additional information. The non-specific immune response, of which phagocytosis is a major element, is important to fish and phagocytic assays are relatively simple to carry out and widely used by researchers in the field of immunotoxicity.

The use of disease challenge or host resistance studies may be supportable as an addition to the industry’s currently supported aquatic biology research. Disease challenges can easily be conducted in existing facilities with equipment already on site. In addition, many immunotoxicity researchers believe that host resistance studies are the only true measure of meaningful immune system suppression.