An indicator species<span> is an organism whose presence, absence or abundance reflects a specific environmental condition. </span>Indicator species<span> can signal a change in the biological condition of a particular ecosystem, and thus may be used as a proxy to diagnose the health of an ecosystem.
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T cells. When HIV arrives in the lymph nodes – around 24 to 48 hours after exposure – they activate other immune cells, such as CD4 t-cells, HIV's primary target.
The enzyme catalyzes (speeds up) the rate at which the substrate is broken down.
