Infection Immunity Immunopathogenesis

Having discussed how both the innate and specific immune responses develop, we will now examine how they act to prevent or overcome infection. Every day we are exposed to an enormous number of bacteria, viruses, fungi and parasites. However, most of us are usually quite healthy. Relatively few of these organisms cause infection but when they do so, they may cause tissue damage in the host. If these organisms are allowed to grow unchecked, they will cause the death of the host either directly or indirectly. However, most infections do not have such a terminal outcome and in people who are generally healthy, infection is usually confined and any tissue damage easily repaired. This ''damage limitation'' is brought about by the cellular and chemical components of the immune system acting together to limit the spread of the infection, kill the microorganisms, and repair the tissue damage.

The ability of different organisms to cause disease (their pathogenicity) varies greatly and is related to how easily they can be controlled by the immune system. This may depend on the evolutionary changes the organism has undergone to evade the immune response, or it may depend on the genetic make-up of the host.

The occurrence of pathological changes in some infectious diseases varies from individual to individual. Chlamydia trachomatis (which is a major cause of infection in the eyes and genital tract in humans) does not cause pathological changes in most people. However in some individuals, severe disease develops, leading to blindness and infertility.

The immune system is designed to prevent infection but it may contribute to the clinical signs and symptoms of the disease, be solely responsible for the clinical disease, may help, or even increase, infection, or be affected by the pathogen leading to long-lasting immunosuppression. The purpose of this

Immunology for Life Scientists, Second Edition. Lesley-Jane Eales. © 2003 John Wiley & Sons, Ltd: ISBN 0 470 84523 6 (HB); 0 470 84524 4 (PB)

chapter is to bring together all the information presented in the previous chapters to illustrate the role of the immune system during infection. We shall examine the relative contribution of both non-specific and specific defence mechanisms in overcoming infection and in causing the tissue damage that occurs in some cases.

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