Clinically available for small number of pathogens (e.g. Mycobacterium tuberculosis). May be useful in determining cellular response to vaccines in antibody-deficiency patients.Less standardized than humoral assays without clear immune correlates of protection.Production of IFN-γ may not reflect utility e.g. in patients lacking IFN-γ receptor, or IL-12p40/STAT1 defectsImmune correlates of protection are not adequately established for cellular assays at present. This evidence concerns the gene IFNG and agammaglobulinemia.