**6.2 Evaluation of cellular immunity**

Delayed-type hypersensitivity (DTH) skin test is commonly used to screen whether the patient has intact cell-mediated immune response. A positive DTH skin test generally rules out the possible defect of cellular immunity [36]. Nevertheless, the test requires that individuals must have sufficient prior exposure and sensitization to the testing antigen; therefore, it may not be suitable for infants and young children. Quantitation of T-lymphocytes (CD3, CD4, CD8) in peripheral blood is able to indirectly reflect the aberrant cellular immunity and can be easily performed by flow cytometry. More specialized T-cell function tests would provide in-depth information in immune system, which include the assessment of lymphocyte proliferation in response to stimulus such as mitogens (e.g., phytohemagglutinin, ConA, and PMA), or specific antigens (e.g., candida). Furthermore, in vitro measurements of intra- and/ or extracellular cytokine responses (e.g., interleukin 2, interferon-gamma, BAFF, and TNF) are informative for the investigation of T- and B-lymphocyte regulation [37].

### **6.3 Evaluation of phagocytic function**

Leukocyte count and differential can assess the phagocytic disorders such as congenital agranulocytosis or cyclic neutropenia. Phagocytic function can be indirectly

assessed by traditional nitroblue tetrazolium (NBT) assay, which measures phagocytic cells' killing capability in response to an oxidative burst. More recently, a simpler dihydrorhodamine 123 (DHR) assay based on flow cytometry has replaced NBT test for assisting the diagnosis of chronic granulomatous disease (CGD), Rac2 deficiency, and complete myeloperoxidase deficiency [38, 39]. Other complicated in-vitro functional methods such as the measurement of directed cell movement (chemotaxis), ingestion (phagocytosis), and intracellular killing (bactericidal activity) are available in some specialized laboratories [40].
