Infection and immunity: written for the Wightman Prize in Clinical Medicine, 1986

The interaction between man and microorganism is complex. The spectrum of disease that results when a microbe gains access to the human body ranges from asymptomatic infection to fulminating disease. Central to this interaction are two factors: the virulence of the microorganism, and the competence of the host defences. The most virulent infections can cause severe, if not fatal illness, although active intervention by the clinician may modify the outcome. However the host defences will overcome the majority of infections. These defences comprise the two arms of immunity: the specific and non -specific defences. The complement cascade and phagocytic cells comprise the non -specific elements, often of importance in the early stages of infection. Humoral and cell mediated immunity, the specific elements directed against a given microorganism, take several days before reaching their full effectiveness and so tend to play a greater role in the later stages of infection.The six cases presented here illustrate various aspects of infection and immunity. The first, a viral infection for which there is no specific treatment, represents the natural history of many infections. Invasion of the host tissues occurs, damage being caused partly by a direct cytopathic effect of the virus, but also by non -specific damage mediated by the immune response. The second case is an infection by a highly virulent organism. Without treatment this infection would be fatal. However given early diagnosis and treatment, a complete recovery might be expected. Organisms of low virulence do not cause disease unless the host defences are compromised, so allowing invasion to occur. The third case illustrates this. The next case is another infection by an organism that favours the compromised host. Untreated it has a high mortality; with direct intervention medical treatment can alter this outcome.The final two cases are examples of autoimmunity. Although the immune system plays a critical role in combatting infection, occasionally it is directed against the host and disease results. Two different infections are seen to initiate this process, pathology resulting in different organs.Although the great scourges of the past such as smallpox, diptheria and polio are unlikely to be major problems in Britain in the future, infections continue to present the clinician with challenging problems. New diseases are recognised in the form of Legionella pneumophilia and the Human T Cell Leukaemia Virus, requiring novel approaches to diagnosis and therapy. The spread of drug resisistance amongst bacteria neccessitates the development of new antibiotics. The increasing number of patients with compromised immune function allows organisms, previously of little importance as pathogens, to cause significant disease.Man's immune system will continue to be a major determinant to the outcome of infection. The field of immunomodulation has enormous potential in terms of therapy, but is as yet little exploited. The immune system might be enhanced by the use of lymphokines such as the interferons or interleukins, both of which can be produced in large quantities by recombinant DNA technology. Synthetic agents might also be capable of boosting the immune response: inosine is one drug for which such claims have been made. Alternatively, developments in cell culture technique might allow the removal of host immune cells, the selection and proliferation in vitro, of cells specifically directed against an invading microorganism, and the subsequent reinfusion of greatly increased numbers of immunocompetent cells. By these measures a feeble immune system might be aided, so overcoming an otherwise fatal infection. There is therefore much scope for research and development in the field of infectious diseases