IMMUNOLOGY - Roitt I. - Mir 2000

Chapter 1. General properties of the immune system

DEFENCE AGAINST EXTRACELLULAR AND INTRACELLULAR INFECTION

Immunological responses to extracellular and intracellular infections differ fundamentally. In response to pathogens that multiply extracellularly, The Immune System aims to destroy the microorganisms themselves and neutralize the Toxic effects of their metabolic products. Conversely, when reacting to microbes that replicate within Cells, two alternative mechanisms are possible: cytotoxic action by T lymphocytes, leading to the destruction of infected cells, or activation of these cells by T lymphocytes to destroy the microbes residing within them. The latter occurs, for instance, when T lymphocytes release cytokines that stimulate macrophages to destroy engulfed microbes.

Since many pathogens exhibit both intracellular and extracellular stages of infection, different mechanisms of the Immune Response become effective at various stages. For example, poliovirus spreads via the bloodstream to infect target cells. At this early stage of infection, pathogen-specific Antibodies are particularly effective. However, to clear an established infection, every virus-infected Cell in the body must be eliminated by cytotoxic T lymphocytes. Consequently, The Role of antibodies is to limit the spread of infection and prevent reinfection by the same virus, whereas cytotoxic T cells are crucial for targeting infected cells (Fig. 1.18). This must be taken into account when designing effective Vaccines.

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Fig. 1.18. Different types of infection require distinct effector mechanisms of the immune response, as illustrated here using a viral infection as an example. Antibodies and Complement can block the extracellular phase of the viral life cycle and promote viral phagocytosis. Interferons released by infected cells can signal uninfected cells to enter an antiviral state. Viruses replicate only within living cells. Cytotoxic T cells can recognize and destroy infected cells before massive viral Replication takes place.