Although free IAA is the biologically active form of the hormone but toxic in high concentration, the vast majority of auxin in plants is found in a covalently bound state are considered hormonally inactive.

IAA has been found to be conjugated to both high- and low-molecular-weight compounds.

Low-molecular-weight conjugated auxins include esters of IAA with glucose or myo-inositol and amide conjugates such as IAA-N-aspartate.

High-molecular-weight IAA conjugates include IAAglucan

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#Unit 4. Cell Communication and Cell Signaling

The inhaled bacilli are ingested by alveolar macrophages in the lung and inhibiting formation of phagolysosomes

CD4+ T cells are activated within 2 to 6 weeks after infection.

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#Unit 4. Cell Communication and Cell Signaling

These cells wall off the organism inside a granuloma called a tubercle. The localized concentrations of lysosomal enzymes in these granulomas can cause extensive tissue necrosis.

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#Unit 4. Cell Communication and Cell Signaling

Diphtheria

Natural infection with C. diphtheriae occurs only in humans and is spread by respiratory droplets. 

The organism colonizes the nasopharyngeal tract and causes little tissue damage, with only a mild inflammatory reaction. 

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#Unit 4. Cell Communication and Cell Signaling

Virulence is due to its potent exotoxin, which destroys the underlying tissue and results in heart, liver, and kidney damage, as well as to suffocation following formation of a tough fibrous membrane in the respiratory tract

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#Unit 4. Cell Communication and Cell Signaling

Malaria (Parasitic)

Sporozoites enter the bloodstream by mosquito.

The sporozoites migrate to the liver, where they multiply, transforming liver hepatocytes into giant multinucleate schizonts, which release thousands of merozoites into the bloodstream. 

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#Unit 4. Cell Communication and Cell Signaling

The merozoites infect red blood cells, which eventually rupture, releasing more merozoites.