ABSTRACT
We have seen how hypersensitivity to harmless environmental antigens leads to acute or chronic disease depending on the nature of the antigen and the frequency with which it is encountered (see Chapter 14). In this chapter we consider a related set of chronic diseases, ones caused by adaptive immune responses that become misdirected at healthy cells and tissues. Such diseases are known as autoimmune diseases, with more than 100 different types being clinically described. Like IgE-mediated allergies, autoimmune diseases are more common in the affluent, industrialized countries. Their frequencies, now 5-10%, have increased like the allergies and they, too, are attributed to recent and ongoing changes in human habits and lives, as invoked in the hygiene hypothesis (see Figure 14.8, p. 407). Autoimmune diseases vary widely in the tissues they attack and the symptoms they cause. Some focus on a particular organ or cell type, others act systemically. For most autoimmune diseases the incidence differs between females and males, with females being more commonly affected.
