The host defense consists of innate, or nonspecific, immunity and adaptive immunity. Innate immunity is present at birth and has 4 components: mechanical barriers such as skin and mucous membranes, secretion of chemicals and enzymes, phagocytosis, and inflammation. Adaptive immunity is acquired immunity that recognizes antigens after exposure and generates pathogen-specific response pathways. The immune response can also be categorized in terms of neutrophil defense, cell-mediated immunity (CMI), and humoral immunity. Abnormalities in the immune system predispose individuals to different types of infections depending on the site of the immune defect. This chapter will discuss the mechanisms of primary neutrophil defense, cell-mediated immunity, humoral immunity, strategies for infection prevention, and the types of commonly seen infections.
PRIMARY NEUTROPHIL DEFENSE
Polymorphonuclear neutrophils (PMNs) are major phagocytes for host immune response. They make up the majority of circulating white blood cells but migrate into tissue in response to a pathogen. Neutrophils kill microbes by engulfing the organism and using reactive oxygen metabolites and digestive enzymes. Neutrophil dysfunction can be qualitative or quantitative. Qualitative dysfunction is usually seen in children and includes abnormalities in (1) diapedesis, the mechanism by which PMNs leave the intravascular space via endothelial channels; (2) chemotaxis, which is the movement of the PMN to the site of infection; (3) ingestion; and (4) intracellular killing (via oxygen-dependent or oxygen-independent).1,2
Quantitative defects are categorized by the absolute neutrophil count (ANC), which is calculated by multiplying the percent of neutrophils by the total white blood cell (WBC) count. Neutropenia is defined as an ANC less than 1500 cells/µL,3,4 while severe neutropenia is less than 500 cells/µL. Neutropenia may be due to benign ethnic neutropenia, which is rarely less than 1200 cells/µL, congenital neutropenias, and acquired neutropenia. Acquired neutropenia can be caused by hematologic conditions, including malignancy and myelodysplastic syndromes, medications, infections, autoimmune, dietary deficiencies, such as vitamin B12, folic acid, or copper, or paroxysmal nocturnal hematuria.5 Neutropenic patients often present with fever, which is defined as single oral temperature greater than 38.3°C (101°F) or a temperature greater than 38°C (100.4°F) sustained over 1 hour.3 Rectal temperatures are generally avoided in neutropenic patients to prevent bacterial translocation into tissues. Patients with neutropenia are at increased risk of bacterial and fungal infections.3
Patients with prolonged (> 7 days) and profound neutropenia (ANC ≤ 100 cells/µL) are at high risk of infection and should be admitted to the hospital for work-up and empiric intravenous (IV) antibiotics when they present with signs and symptoms suggestive of infection. Lower-risk patients with brief (≤ 7 days) neutropenia who are clinically stable may be eligible for outpatient management and oral therapy.3 In cancer patients with neutropenic fever, risk stratification is determined via Multinational Association for Supportive Care in Cancer Risk-Index Score (MASCC score) using the patient’s burden of illness (clinical symptoms), comorbidities, outpatient status at ...