Components of the Immune System 120
immune Responses to iViral Infection 129
Recovery from Viral Infection 131
Immunity to Reinfection 133
Passive Immunity 134
In response to the constant threat of invasion by microorganisms and viruses, vertebrates have evolved an elaborate set of defensive measures, called, collectively, the immune system. During the initial encounter with a virus, the immune system of the host recognizes as foreign certain viral macromolecules (proteins, carbohydrates) called antigens, which elicit several kinds of responses to eliminate the virus and to prevent reinfection. Cells of the humoral immune system (B lymphocytes) respond to an antigenic stimulus by producing and secreting specific immunoglobulins called antibodies; cells of the cell-mediated immune system (T lymphocytes) respond by secreting several cytokines which regulate the immune response by coordinating the activities of the various types of cells involved. These lymphocytes bear highly specific receptors that enable them to interface with discrete sites on the virion or on viral peptides, known as antigenic determinants or epitopes This specific recognition event triggers a wide range of effector processes that attack and remove the invading virus and virus-infected cells. The resulting cascade of cell-cell interactions and cytokine secretion amplifies the immune response to match the scale of the virus infection and, in addition, establishes a long-lived memory that enables the immune syslem to respond more quickly to any reinfection that may occur later in life.
The immune response terminates many viral infections before much damage has been done, resulting in mild or even subclinical infections. This chapter deals with the role of the immune response in recovery from viral infection and resistance to reinfection. Later chapters address situations where the immune system does not function so effectively: where the im mune response is actually harmful, causing tissue damage in vital organs (Chapter 9), or where the virus evades the immune system and establishes a persistent infection (Chapter 10).
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