What typically causes allergic effects in individuals exposed to substances?

The correct answer centers on the understanding of allergic reactions, which often develop after repeated exposure to a particular substance. Allergies occur when the immune system becomes sensitized to a specific allergen, leading to an inappropriate immune response upon subsequent exposures. This biological mechanism explains why only some individuals experience allergic reactions, particularly after multiple exposures.

People may not react after the first exposure because the immune system needs time to recognize and react to the allergen. It's the subsequent exposures that typically trigger the immune response, resulting in symptoms of an allergic reaction. Therefore, this option accurately reflects the process through which allergies develop over time for certain individuals.

The other options do not encapsulate this key aspect of allergic reactions. Immediate exposure without prior contact suggests that reactions can happen right away for everyone, which misrepresents the nature of sensitization. High doses of chemicals may lead to toxicity or irritation but are not a prerequisite for allergic reactions; rather, even small amounts of an allergen can provoke a response in sensitized individuals. Lastly, environmental factors alone can't account for the individual variability seen in allergic responses, as allergic reactions are primarily dependent on a person's unique immune response to specific allergens rather than external environmental factors alone.