[Susceptibility to infection]

Recurrent infections in individual patients prompt concern regarding an underlying immunodeficiency. It is often difficult to make a distinction between "normal" and abnormally frequent occurrence of infections. Infectious diseases are the result of a series of complicated interactions between host factors, pathogen properties and epidemiological circumstances. This paper reviews the role of non-specific and antigen-specific host factors relevant to the management of a patient with suspected immunodeficiency. The most important effector mechanisms of immune defence are outlined and the consequences of their failure with respect to specific infections are discussed. Analysis of the clinical presentation, pathophysiology and microorganisms involved offers clues as to which part of the immune system is failing in an individual patient. Based on this, further tests will confirm or exclude an underlying immunodeficiency. The paper concludes with an overview of relevant therapeutic and prophylactic options.