Tissue tropism is the range of cells and tissues of a host that support growth of a particular pathogen, such as a virus, bacterium or parasite.^[1][2]

Some bacteria and viruses have a broad tissue tropism and can infect many types of cells and tissues.^[1] Other viruses may infect primarily a single tissue.^[1] For example, rabies virus affects primarily neuronal tissue.

Factors influencing viral tissue tropism include:

• The presence of cellular receptors permitting viral entry.
• Availability of transcription factors involved in viral replication.
• The molecular nature of the viral tropogen or virus surface, such as the glycoprotein, which interacts with the corresponding cell receptor.

The cellular receptors are the proteins found on a cell or viral surface. These receptors are like keys, allowing the viral cell to fuse with or attach itself to a cell. The way that these proteins are acquired is through a similar process to that of an infection cycle.

Tissue tropism develops in the following stages:

• Virus with GPX enters body (where GP - glycoprotein and X is the numeric value given to the GP)
• Viral cell "targets" cell with a GPX receptors
• Viral cell fuses with the host cell and inserts its contents into the host cell
• Reverse transcription occurs
• Viral DNA is incorporated with host DNA via viral enzyme
• Production of RNA and viral protein
• Viral particle is assembled
• Viral particle buds out of the cell, taking a chunk of the cell membrane with it and acquiring a new tissue with all the receptors it needs to continue tissue tropism

Example: HIV has a gp120, which is precisely what the CD4 marker is on the surface of the macrophages and T cells. Thus HIV can enter T cells and macrophages

• Endothelial cell tropism
• Host tropism

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