The immune system is a complex network and inadvertent immunological reactivity can result in HYPERSENSITIVITY or autoimmunity. Nearly every chemical can induce HYPERSENSITIVITY reactions which can affect nearly every organ or tissue of the body. One organ or tissue, however, is often a predominant target. Anaphylactic shock, skin reactions, and cytopenias are the most frequent reactions. HYPERSENSITIVITY reactions can be either immunemediated or non-immune-mediated (pseudoallergy). A number of different mechanisms are involved and risk factors can serve as triggers. Autoimmunity is still a mystery even though autoimmune diseases are relatively common. Spontaneous autoimmune diseases as well as drug- and chemical-induced autoimmune reactions are divided into systemic and organ-specific. Our current understanding of the mechanisms involved in these adverse reactions is very limited.