Array of serum proteins (some of which are enzymes) that become sequentially activated after the first member of the series is activated by either antigen-antibody complexes or microbial products.
Source:
PAC, 2018, 90, 1121. 'Terminology of bioanalytical methods (IUPAC Recommendations 2018)' on page 1137 (https://doi.org/10.1515/pac-2016-1120)