ADA is an enzyme catalyzing the deamination reaction from adenosine to inosine. It is also an essential enzyme of the purine catabolic pathway. ADA presents in all cell types, however, the amount of enzyme differs widely among tissues.
The highest ADA levels in humans are found in lymphoid tissues. ADA acts in proliferation and differentiation of lymphocyte, especially T lymphocyte. It also acts in maturation of monocytes transforming them to macrophage. ADA is a significant indicator of active cellular immunity. For example, deficiency in ADA in humans manifests primarily as severe lymphopenia and immunodeficiency. Ecoline ADA has been proposed to be a useful surrogate marker for TB because it can be detected in body fluids such as pleural, pericardial and peritoneal fluid. The levels of ADA increase in TB because of the stimulation of T cells by mycobacterial antigens. Ecoline ADA is highly specific and sensitive in detecting TB as compared to the other tests.