Abstract
In this chapter, we will discuss about different types of hypersensitivities of the immune system, which are either mediated primarily by IgEs and T H 2 cells (type I), IgGs and IgMs (type II), immune complexes (type III) and T cells (type IV). While type I hypersensitivity causes different forms of allergy, the basis for distinct autoimmune diseases are type II–IV hypersensitivities. The underlying mechanisms for all hypersensitivities, is the failure of mechanisms responsible for maintaining self-tolerance in B cells, T cells or both. We will outline the effector mechanisms, responsible for tissue injury and disease in the context of these hypersensitivities, which are based on genetic susceptibility and environmental triggers, such as infections. Organ-specific autoimmune diseases, such as T1D and MS are distinguished from those that are systemic, such as rheumatoid arthritis and SLE. We will summarize these principles in the equilibrium model of immunity.