Human populations can exist at equilibrium densities in one of two ways:

1. High birth and high death rates

2. Low birth and low death rate

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Mechanisms of humoral and cell-mediated immune responses

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Many Viruses Are Neutralized by Antibodies

Antibodies are particularly effective if they are localized at the site of viral entry into the body.

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Cell-Mediated Immunity against Virus

Both CD8+ TC cells and CD4+ TH1 cells are required components of the cell-mediated antiviral defense.

Activated TH1 cells produce IL-2, IFN-ϒ, and tumor necrosis factor-α.

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IFN-ϒ acts directly by inducing an antiviral state in nearby cells.

IL-2 acts indirectly by assisting the development of cytotoxic T lymphocyte(CTL) precursors into an effector population.

Both IL-2 and IFN-ϒ activate NK cells.

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Viruses Employ Several Different Strategies to Evade Host Defense Mechanisms

Hepatitis C virus, inhibiting the action of PKR.

Herpes simplex virus TAP.