As a result of the huge number of potential toxicants and the multitude of biological
structures and processes that can be impaired, there are a tremendous number of possible
toxic effects. Correspondingly, there are various pathways that may lead to toxicity (Fig. 2-
1). The most complex path to toxicity involves more steps. First, the toxicant is delivered to
its target or targets (step 1), after which the ultimate toxicant interacts with endogenous
target molecules (step 2a), triggering perturbations in cell function and/or structure (step 3),
which initiate repair mechanisms at the molecular, cellular, and/or tissue levels as well as
adaptive mechanisms to diminish delivery, boost repair capacity and/or compensate for dysfunction (step 4). When the perturbations induced by the toxicant exceed repair and
adaptive capacity or when repair and adaptation becomes malfunction, toxicity occurs.
Tissue necrosis, cancer, and fibrosis are examples of chemically induced toxicities whose
development follow this 4-step course.