Protein antigens structural complexity

The reactions that take place in the complement system can be initiated in several ways. During the early phase of infection, lipopoly-saccharides and other structures on the surface of the pathogens trigger the alternative pathway (right). If antibodies against the pathogens become available later, the antigen-antibody complexes formed activate the classic pathway (left). Acute-phase proteins (see p. 276) are also able to start the complement cascade lectin pathway, not shown). 

Noncovalently bound adducts can also be observed, involving enzyme and substrate, protein and ligand, protein and protein, antigen and antibody that can influence the biological activity of the cells [26]. The study and the detection of noncovalent complexes has the benefit of enormous advantages using ESI-MS because it requires a lower amount of material and a shorter time to provide structural information. 

The structures of oligosaccharides linked to these proteins can be complex and many of them contribute to antigenicity, the ability of the cell surface to elicit an immune response. 

Protein A is a specific protein isolated from the cell wall of Staphylococcus aureus whose characteristic property is the ability to react and to form precipitates with a variety of IgG molecules from several species. This interaction is reminia nt of the formation of antigen-antibody complexes, and has been used to study different aspects of immune response as well as cell surface structure and function For easier detection, Protein A is covalently coupled to fluorescein isothiocyanate (FITC). The commercial preparations of FITC-Protein A contain an avarage of 6 FITC substituent groups per molecule of protein. Such a degree of labeling does not affect the biological properties of the native protein. 

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