Tissue tropism is a term most often used in virology to define the cells and tissues of a host

Host (biology)

In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...

 which support growth of a particular virus. Bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 and other parasites may also be referred to as having a tissue tropism.

Some viruses have a broad tissue tropism and can infect many types of cells and tissues. Other viruses may infect primarily a single tissue. For example Rabies

Rabies

Rabies is a viral disease that causes acute encephalitis in warm-blooded animals. It is zoonotic , most commonly by a bite from an infected animal. For a human, rabies is almost invariably fatal if post-exposure prophylaxis is not administered prior to the onset of severe symptoms...

 virus affects primarily neuron

Neuron

A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...

al tissue, and Hepatitis

Hepatitis

Hepatitis is a medical condition defined by the inflammation of the liver and characterized by the presence of inflammatory cells in the tissue of the organ. The name is from the Greek hepar , the root being hepat- , meaning liver, and suffix -itis, meaning "inflammation"...

 primarily affects liver tissue.

Influencing factors

Factors influencing viral tissue tropism include:

• The presence of cellular receptors permitting viral entry.
• Availability of transcription factor
  Transcription factor
  In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...
  
  s involved in viral replication.
• The molecular nature of the viral tropogen
  Tropogen
  The protein on the outer surface of a virus that combines with the cellular receptor for the virus. The interaction between the viral tropogen and the cellular receptor is quite specific. This interaction may play a role in determining tissue tropism and host range.An example of a tropogen is...
  
  .

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 a cell, or attach itself to a cell. The way that these proteins are acquired is through similar process to that of an infection cycle.

How 'Tropic' Tissue is acquired

Steps of Infections Cycle (how Tissue Tropism works)

• Virus with GPX enters body (where GP - glycoprotein
  Glycoprotein
  Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...
  
   and X is the numeric value given to the GP)
• Viral Cell 'targets' cell with a GPX receptors
• Viral Cell fuses with the cell and dumps its contents into it
• Reverse Transcription
  Transcription (genetics)
  Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
  
   occurs
• Viral DNA
  DNA
  Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
  
   is incorporated with host DNA
  DNA
  Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
  
   via Viral Enzyme
  Enzyme
  Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
  
  
• Production of RNA
  RNA
  Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
  
   and Viral Protein
  Protein
  Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
  
  
• Viral particle is assembled
• Viral particle buds out of the cell taking a chunk of the cell membrane with it 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