Free circulating antibodies, precipitins, which belong to the immunoglobulin G class, cause these reactions. The complement system is activated when these antibodies react with locally introduced antigens to produce an antigen-antibody complex. This activation of complement, together with complement-induced granulocyte accumulation and release of histamine via anaphylatoxins, results in tissue damage, an Arthur’s reaction. Allergic alveolitis is an example of such an immune complex reaction: inhalation of antigens such as molds, plant or animal antigens results in tissue changes, giving rise to symptoms such as coughing, dyspnoea, fever and cyanosis. If the allergic condition is allowed to persist the tissue damage may be irreversible. Persistent infections and autoimmunity could cause other categories of immune-complex diseases.
Another example of a type III reaction is serum sickness: antigen introduction, in the form of a foreign protein, results in the formation of IgG antibodies. The antigen-antibody complex formed from circulating antigen and a precipitated antibody is deposited in different tissues (e.g. lung), resulting in tissue damage. It has been shown that the immune complex which is formed with an excess of antigen is the most damaging. The symptoms of serum sickness can partly be of a general nature such as fever, pain in the joints, skin changes or swelling of the lymph glands, but they can also be localized to organs such as the heart or the kidneys.
Renewed introduction of allergen to an individual who has previously presented the symptoms of serum sickness will result in a rapid onset of symptoms, which as a rule will be more severe than in the first instance. The rapid course of events in accelerated serum sickness is due to the fact that the antibody production by the booster effect is much faster than the initial production. Frequently, the reintroduction of the antigen is accompanied by an immediate anaphylactic shock for which a type I reaction may be responsible.
Reference: Roitt I, Brostoff J, Male D. Immunology.
5th edition. Hypersensitivity – Type I. London:
Mosby, 1998:301 (1).