Deficiencies in any of the defense mechanisms of the host can lead to severe microbial infections; these are of clinical relevance. Broad up-to-date knowledge in this field allows identification of many unspecific as well as highly specific defense reactions involved in the struggle against infectious diseases. On the other hand, protective structures on the microbial cell surfaces have been adapted and improve the counterpart's chances of survival. In particular, it has been considered that the great diversity of the bacterial envelopes not only determines the anatomical location of the tissue injury but also induces activation of distinct parts of the complex defense system. The specific defense mechanism, whose most prominent constituent is provided by the antiboDeficiencies in any of the defense mechanisms of the host can lead to severe microbial infections; these are of clinical relevance. Broad up-to-date knowledge in this field allows identification of many unspecific as well as highly specific defense reactions involved in the struggle against infectious diseases. On the other hand, protective structures on the microbial cell surfaces have been adapted and improve the counterpart's chances of survival. In particular, it has been considered that the great diversity of the bacterial envelopes not only determines the anatomical location of the tissue injury but also induces activation of distinct parts of the complex defense system. The specific defense mechanism, whose most prominent constituent is provided by the antiboBacteria and Complement A Historical Review.- The Complement System: Activation and Control. With 3 Figures.- The Role of the Classical Pathway for the Bactericidal Effect of Normal Sera Against Gram-Negative Bacteria. With 27 Figures.- Evidence for Direct Binding of the First Component of Complement, C1, to Outer Membrane Proteins from Salmonella minnesota. With 7 Figures.- Differences in Attachment and Phagocytosis of Salmonella minnesota Slc+