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The body's defence against foreign substances and organisms is known as the immune system, and immune reactions play an important part in many work-related diseases. The immune system is complex and relies on the work of many different types of cells which are found in bone marrow, thymus, spleen, lymph nodes, in the lining of the gut and respiratory tract, and in the skin.

The action of some cells is very general, but others respond to specific substances. One of the hallmarks of this specific immunity is that "memory" develops, so that when the body comes into contact with the same substance a second time the response is faster and stronger. Specific immunity is divided into antibody-mediated and cell-mediated systems, and substances that can cause immune responses are called antigens; many of them are proteins.

Hypersensitivity or allergic reactions can be classified into four distinct types: Type I, anaphylactic or immediate hypersensitivity reactions in which symptoms occur within minutes of exposure, Type II, cytotoxic reactions in which symptoms appear within hours, Type III, immune complex reaction, and Type IV, cellular immunity or delayed-type hypersensitivity in which symptoms develop 24-48 hours after exposure.

The most common allergies at work affect the skin and respiratory tract. Common causes of allergic contact dermatitis include latex (in surgical gloves), formaldehyde, rubber additives, nickel or chromium compounds, bactericides and fungicides, adhesives and sealants, plants and wood, and hairdressing chemicals.

Common causes of allergic asthma and rhinitis include latex, flour, mites, enzymes, di-isocyanates, metals and metal compounds, and certain drugs.

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