Immune responses defence against viruses

The complement system belongs to the non-specific humoral defence system. It plays an important part in antigen processing and is involved in the defence against viruses and tumors. Activation of complement leads to increased destruction and lysis of cells that possess a lipoprotein membrane, i.e. bacteria, viruses, and tumor cells. Conversely, complement is partly responsible for some inflammatory processes that occur in a state of hyper-reactivity and is able to eliminate immune aggregates. The complement system can be subdivided into the classical and alternative activation pathway. The classical pathway is dependent on antibodies (IgM and IgG), whereas the alternative pathway can be activated by microorganisms on their own or in combination with IgA antibodies. 

Two major advantages stem from the use of live vaccines. Firstly, the immunization mimics a natural infection such that only a single exposure is required to render an individual immune. Secondly, the exposure may be mediated through the natural route of infection (e.g. oral) thereby stimulating an immime response that is appropriate to a particular disease (e.g. secretory antibody as primary defence against poliomyelitis virus in the gut). 

---