The immune system has evolved to combat the constant threat of tissue invasion by microorganisms. If, however, the immune system is directed against innocuous antigens or tissue antigens, the same immune responses that are vital for defence against microorganisms can result in autoimmune disease and allergy. The adaptive immune response is reliant on the properties of B- and T-cells that enable the response to be powerful, flexible and antigen-specific and exhibit immunological memory. B-cells secrete antibodies that are effective against extracellular bacteria and their toxins, whereas CD8 T-cells are adept at neutralizing virally infected cells. CD4 T-cells, also known as Th-cells, do not directly neutralize invading pathogens; instead, they interact with other cells (e.g. macrophages and B-cells) to direct a coordinated, antigen-specific immune response against microorganisms. CD4 Th-cell differentiation can be usefully considered to be either Th1- or Th2-biased. Th1-biased immune responses are characterized by IgG production and macrophage activation; such a response is vital for defence against extra/intracellular bacteria, fungi and some parasites. Conversely, inappropriate Th1-biased immune responses underlie autoimmune diseases. Th2-biased immune responses are characterized by IgE secretion, mastocytosis and eosinophilia, and, although useful in eliminating helminth infestations, such responses underlie the allergic diseases of asthma, eczema and hay fever.

This chapter is an attempt to provide an outline of the immune system; inevitably space constraints have necessitated oversimplification and the omission of some aspects. The major aspects of the immune system have been covered but if readers require further detail, they should consult one of the many readily available large immunological textbooks.

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