Immune Response to Viruses: Antibody-Mediated Immunity

The notion about immunity to disease arose from the observation that those who recovered from an apparently contagious disease became resistant to a subsequent similar sickness. Much later it was shown that immunity is transferable by serum. The active serum components were identified to be immunoglobulins (Ig's), called antibodies. The enormous diversity of antibodies specific for distinct viruses, pathogens, and many other antigens is explained by clonal selection, whereby specific B-lymphocyte receptors recognize a particular antigen. The selected B-cells are triggered to undergo replication. A process of further cell differentiation and maturation ensues, leading to secretion of antibodies with high binding affinity toward the triggering antigen. Genes coding for the variable regions (involved in antigen binding) of Ig's are inherited as sets of gene fragments joined to form a complete gene in individual B-cells. This process and further hypermutations ensure the synthesis of diverse high affinity antibodies. The antibodies consist of pairs of light (L) and heavy (H) polypeptide chains. Variations in the constant portion of H-chains lead to production of Ig isotypes (IgM, IgA, IgD, IgE, and IgG (further subdivided into IgG1, IgG2, IgG3 and IgG4)), each having distinct effector functions. Host exposure to viruses leads to the production of antibodies with more than one specificity. Only some of these antibodies, recognizing so-called virus neutralization epitopes, diminish or eliminate virus infectivity. Other virus-specific antibodies play auxiliary roles or are ineffective. Sometimes antibodies cause enhancement of viral diseases or play a role in evasion of the immune system. Many antiviral immunoglobulins are being used for short-term pre-exposure prophylaxis or therapy. Long-term protective effects can be accomplished only by antibodies elicited by successful vaccination relying on the phenomenon of immunological memory. T-lymphocytes play a major role in initiating and maintaining immunity against subsequent virus exposure. Antibodies are one of the essential features of antigen triggered adaptive immunity. Initial early defense against viruses is provided by components of innate immunity which evolutionarily precedes adaptive immunity, and remains an essential part of defense against pathogens in humans.