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FIGURE 10-1Model of the competent immune system depicting sites of potential effects of the major components by toxic factors. SOURCE: National Research Council, 1992.

a variety of agents that are usually not pathogenic. Age, poor nutrition, and stress (physiologic and psychologic) can exacerbate the development of such immunologic disease (Golub, 1987).

Xenobiotics can also act as sensitizers to stimulate the immune system as antigens by provoking a substantial immune response that leads to hypersensitivity. Immunologic tissue damage can result from activation of the cellular and biochemical systems of the host. The interactions of an antigen with a specific antibody or with effecter lymphocytes trigger the sequence of humoral and cellular events to produce the pathophysiologic effects that lead to tissue injury or disease (Vos, 1977).

Chemicals that suppress bone marrow function can affect reserves of stem cells that are needed for cell replacement. Blood line cells are derived from  stem  cells, which can develop into many cell types (pluripotent) and which, in adult humans, are primarily in bone marrow. Within the marrow  microenvironment, these self-renewing cells mature into committed progenitors, which are in peripheral cells. Stem cells often appear to be sensitive targets for therapeutic and environmental toxicants, most likely because of their rapid proliferation. Xenobiotics or various drugs that are toxic to the myelocytes of the bone marrow can cause profound immunosuppression due to loss of stem cells.

Animal Studies

In a review of the literature, a distinct impression arises that sulfur mustard has been consistently observed to cause pathogenic states of



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