Infections in the immune-compromised host: secondary immunodeficiency

The immunocompetent defence against infection comprises a multifaceted and coordinated armoury of physical barriers, and innate and adaptive immunity. Certain medical conditions and treatments can compromise either single or multiple components of this defence and provoke susceptibility to pathogens rarely encountered in immunocompetent hosts. When evaluating such patients, it can be helpful to consider things from both a ‘pathogen-focused’ and a ‘host-focused’ perspective. Understanding geographical and healthcare contact can help inform which bacterial, viral, fungal and parasitic pathogens the patient has been exposed to, and screening swabs/serological tests can further clarify this risk. The specific clinical scenario at hand always dictates which infections should be considered but no pathogen is linked to a single clinical syndrome. Both the likely infection and its severity are contingent upon which component of immunity has been compromised, to what degree and for how long. The time that has elapsed since transplantation has a strong influence over which infections are plausible in the recipient, whereas for patients with HIV, it their CD4+ count. The overall host vulnerability to infection, however, results from an interplay between these direct causes of immunodeficiency, other co-morbidities and iatrogenic factors such as catheters and surgical interventions.