Our previous discussions of the immune response centered primarily around its numerous beneficial effects. The precisely coordinated system that recognizes and destroys invading microbes is clearly protective, but it also presents another side—a side that promotes rather than prevents disease. In this chapter, we survey immunopathology, the study of disease states associated with overreactivity or underreactivity of the immune response. In the cases of allergy and autoimmunity, the tissues are innocent bystanders to immune attacks that do not distinguish between one’s own cells and foreign antigens. In grafts and transfusions, a recipient reacts to the foreign tissues and cells of another individual. In immunodeficiency diseases, immune function is incompletely developed, suppressed, or destroyed. Cancer falls into a special category, because it is both a cause and an effect of immune dysfunction. As we shall see, one fascinating by-product of studies of immune disorders has been our increased understanding of the basic workings of the immune system.
Overreactions to Antigens: Allergy/ Hypersensitivity
The term allergy means a condition of altered reactivity or exaggerated immune responsiveness that is manifested by inflammatory symptoms. Although it is sometimes used inter changeably with hypersensitivity, some experts use the term allergy in reference to immediate reactions (hay fever, asthma) and use hypersensitivity in reference to other forms of over reactions to antigens (contact dermatitis). Allergic and hypersensitive individuals are acutely sensitive to repeated contact with antigens called allergens that do not notice ably affect other “normal” individuals. Although the general effects of these immune pathologies are usually detrimental, we must emphasize that they involve the very same types of immune reactions as protective immunities. These include humoral and cell-mediated actions, the inflammatory response, phagocytosis, and complement. Such an association means that all humans have the potential to develop allergy or hypersensitivity under particular circumstances.
Originally allergies and hypersensitivities were defined as either immediate or delayed, depending upon the time lapse between contact with the allergen and onset of symptoms. Subsequently they were differentiated as humoral versus cell-mediated. But as information accumulated on these types of dysfunctional immune responses, it became evident that, although useful, these schemes over simplified what is really a very complex spectrum of reactions. The most widely accepted classification, first introduced by immunologists Philip Gell and Robert Coombs, includes four major categories of immune system pathologies involving overreactions to antigens: type I (“common” allergy and anaphylaxis), type II (IgG- and IgM-mediated cell damage), type III (immune complex diseases), and type IV (delayed hypersensitivity) (table 1). In general, types I, II, and III involve a B-cell–immunoglobulin response, and type IV in volves a T-cell response. The antigens that elicit these reactions can be exogenous, originating from outside the body (microbes, pollen grains, and foreign cells and proteins) or endogenous, arising from self tissue (autoimmunities).
Table1. Classification of Immunopathologies Caused by Overreactions to Antigens
One of the reasons allergies and hypersensitivities are easily mistaken for infections is that both involve damage to the tissues and thus trigger the inflammatory response, as described in chapter 14. Many symptoms and signs of inflammation (redness, heat, skin eruptions, edema, and granuloma) are prominent features of allergies and are caused by some of the same chemical mediators.
