Microbial and mammalian cell surfaces can activate the alternative pathway in the absence of specific antigen– antibody complexes. This nonspecific activation offers a major physiologic advantage, because host protection can be generated before the induction of an antibody-mediated immune response. Factors capable of activating the alternative pathway include inulin, zymosan, bacterial polysaccharides, and aggregated immunoglobulins IgG4, IgA, and IgE. Proteins involved in this reaction include properdin, factor D, factor B, and C3.
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