2.5: Toxic Reactions: Overview

When toxic substances penetrate the human body, they disseminate to various tissues, undergoing metabolic changes. This process yields reactive metabolites that may covalently bind with specific target molecules, resulting in toxicity.

Toxicity falls into two primary categories: local and systemic.

Local toxicity appears at the exposure site, such as protein denaturation caused by caustic substances.

In contrast, systemic toxicity requires the toxic agent's absorption and distribution, potentially disrupting a specific biochemical pathway.

Certain toxins can trigger both local and systemic effects. For example, tetraethyl lead can cause skin irritation upon contact, a local effect, and disrupt the central nervous system when absorbed into the bloodstream, which is a systemic effect.

The severity and reversibility of toxic effects depend significantly on the affected organ. Damage to the central nervous system is typically irreversible due to the limited regenerative capacity of brain neurons. On the other hand, injuries to organs like the liver might be reversible because of the organs inherent regenerative abilities.

The onset of toxic effects varies from immediate to delayed. For instance, an excess of acetylcholine at synapses due to malathion-mediated inhibition of acetylcholinesterase can quickly induce symptoms. On the other hand, asbestos may have long latency periods before causing diseases like mesothelioma, a cancer type that affects specific tissue linings.

Toxic chemicals entering the body get distributed to tissues where they may be metabolized. The reactive metabolites may interact covalently with target molecules to produce toxicity.

Local toxicity occurs at the site of exposure. An example is the denaturation of proteins by caustic substances.

Systemic toxicity usually requires absorption and distribution of the toxicant which may function by interrupting a specific biochemical pathway.

Some toxic effects may be both local and systemic. For example, exposure to tetraethyl lead can cause skin irritation, and its absorption into the circulation can affect the CNS.

Injuries to the CNS are irreversible due to the limited capacity of brain neurons to regenerate. Injuries to organs like the liver may be reversible due to its regenerating ability.

Most toxic effects are quick. For instance, excess acetylcholine at synapses, due to malathion-mediated inhibition of acetylcholinesterase, produces immediate symptoms.

Some toxic chemicals, like asbestos, exhibit long latency periods before leading to mesothelioma—a cancer of the linings of certain tissues.