( One ( then ( ( looks ( for ( an ( antigen-antibody reaction ) ) ) ) . ) ) )
(ROOT (S (NP (PRP One)) (ADVP (RB then)) (VP (VBZ looks) (PP (IN for) (NP (DT an) (JJ antigen-antibody) (NN reaction)))) (. .)))
One then looks for an antigen-antibody reaction.
(ROOT (S (NP (DT An) (NN antibody)) (VP (MD can) (VP (VB match) (NP (NP (CD one) (NNS types)) (PP (IN of) (NP (NNS antigens)))))) (. .)))