Whole blood measures include the following:
- a complete blood count with a five-part differential to determine total white cells, total lymphocytes, and total eosinophils; and
- number of CD4+ lymphocytes and the CD4+:CD8+ ratio, by lymphocyte phenotyping. CD4+ lymphocytes are helper–inducer T-lymphocytes, whereas CD8+ cells are suppressor or cytotoxic T-cells. A decrease in CD4+ and a CD4+:CD8+ ratio of less than 1.5 is correlated with immune impairment and increased susceptibility to infection (Bloom et al., 1992). Although lymphocyte phenotyping with a complete panel of major phenotypes is recommended by the ATSDR as the most predictive test for immunotoxicity in animals (Luster et al., 1992, 1993) and can be performed with small samples of whole blood (making the tests amenable to field studies), these tests are not able to evaluate functional status. Lymphocyte subpopulations vary over time and between individuals. In addition, peripheral immune cells and compartments, although easy to sample, may not reflect immune function in the lymphoid tissues where the most significant changes in lymphocyte subpopulations are likely to occur, and samples drawn from peripheral pools may be more reflective of one part of the immune system than the other (humoral versus systemic) (see Susanna Cunningham-Rundles, see Chapter 9). Monoclonal antibodies (Mabs) that identify cell-surface markers associated with T-cell activation (such as CD25, the interleukin-2 (IL-2) receptor) are now becoming available and may alleviate some of the problems mentioned.
Focused Tests of Immune (Deficiency) Function
For individuals whose basic test results provide evidence of immune deficiency, a panel of more specific tests may be performed.
Tests of Serum
Secondary humoral response (IgM and IgG responses to an antigen to which the subject has been exposed previously) can be assessed in samples of serum drawn before and after immunization with the recall antigen, making this a suitable technique for use in the field (Straight et al., 1994). Factors that must be considered include the timing of blood sampling (to take into account variations in immune function resulting from biological rhythms [see Erhard Haus, Chapter 20]) and the background (''before'') antibody titer expected. An additional consideration is the great degree of variability observed in response to immunization, including a fairly high percentage of nonresponders.
Antibody response to primary immunization with a novel antigen may be measured as an alternative. This test requires identification of an antigen to which the subject has not been exposed previously (see Cunningham-Rundles, Chapter 9).