Immunoassays are currently the main analytical technique for quantification of a wide range of analytes of clinical, medical, biotechnological, and environmental significance with high sensitivity and specificity. Miniaturization of immunoassays is achieved using microfluidics coupled with integrated optical detection of the antibody-antigen molecular recognition reaction using thin-film amorphous silicon (a-Si:H) photodiodes. The detection system used consists of an a-Si:H photodiode aligned with a polydimethylsiloxane (PDMS) microchannel. An enzymatic reaction taking place in the microchannel yields a product which is a light-absorbent molecule and hence can be optically detected by the integrated photodiode. Specific antigen-antibody reaction was detected and distinguished from the non-specific reaction.