Classical pathway activation during an adverse response to protamine sulphate. 
A severe anaphylactoid reaction was observed following the i.v.  administration of protamine sulphate in a 66-year-old male undergoing coronary artery catheterization.  Serial measurements of complement factors (C3 and C4) and C3 split products (C3c and C3d) were performed by conventional electro-immunoassays and by double zone rocket immunoelectrophoresis.  A parallel decrease in circulating C3 and C4 concentrations was seen for 1 h, reaching a nadir at 1.3 h. Markedly increased concentrations of C3c and C3d were observed in the first sample examined after exposure to the drug.  C3c and C3d concentrations gradually declined to the normal range within 5.9 h and 21.8 h, respectively.  These observations are discussed in relation to the mechanism of this adverse reaction.