Introduction
This topic covers the laboratory methods used to measure the cellular arms of immunity: phagocytic function of neutrophils and monocytes, the phenotype of blood leukocytes by immunophenotyping, and the functional capacity of lymphocytes and NK cells. It also explains how these results are combined into an immune status and used to monitor patients with immunodeficiency, autoimmune disease, infection, transplantation and malignancy.
Principles of testing cellular immunity
Cellular tests examine living cells, so specimens (usually heparinised or EDTA venous blood) must be processed quickly, typically within hours, and results depend on pre-analytical conditions. Tests fall into three groups: those that measure innate effector function, those that identify cell populations by surface markers, and those that measure how lymphocytes respond when stimulated.
Tests for phagocytosis and oxidative burst
- Phagocytic index: neutrophils or monocytes are incubated with opsonised bacteria, yeast or latex particles, then counted microscopically or by flow cytometry to estimate the fraction of cells that ingest particles and how many particles each takes up.
- NBT test (nitroblue tetrazolium reduction): reduction of the yellow dye to dark blue formazan inside activated phagocytes shows NADPH oxidase activity. Negative or greatly reduced reduction indicates chronic granulomatous disease.
- Dihydrorhodamine (DHR-123) flow cytometry: a more quantitative modern method of measuring the oxidative burst, which also distinguishes carriers and subtypes of CGD.
- Adhesion and chemotaxis: flow cytometry for CD18 and CD11b, and chemotaxis assays, are used when leukocyte adhesion deficiency is suspected.