A cold is the product of viral infection of the upper respiratory tract, usually by a rhinovirus, which literally means "nasal virus." While it's not possible to treat a cold, the body's immune system effectively eliminates most infections within a week or two. Elimination of the virus involves many aspects of the immune system.
Features
Cold viruses, contrary to popular belief, aren't the result of spending time outside in the winter or becoming chilled. Instead, they're spread by infected individuals through contact with virus-containing nasal mucus or saliva, explains Dr. Jacquelyn Black in her book "Microbiology: Principles and Explorations." The cold virus binds to receptors on cells in the back of the nose and infects those cells. Early symptoms such as stuffiness and runny nose are the result of nasal cell damage and death.
Viral Infection
The mechanism by which the immune system becomes aware of a cold virus infection is slightly different from that for bacterial infections. In her book "Human Physiology," Dr. Lauralee Sherwood notes that viruses lack the ability to self-replicate, meaning that they must infect human cells to reproduce. Therefore, instead of looking for replicating viruses in the bloodstream, the immune system looks for signs of infected cells whose reproductive equipment has been taken over by a virus, and which are producing new viral bodies.
Cold Symptoms
Many of the symptoms of a cold are caused not by the virus itself but by the immune system's response to the virus, explains David Proud, Ph.D., in an article published on Science Daily. Cellular distress molecules and irritant chemicals released by infected cells and immune cells cause the body's temperature to increase, producing a fever. Fevers make it harder for pathogens to reproduce in the body, but also lead to the headaches and muscle or joint pain associated with colds. Stuffy noses and chest congestion are the result of excess mucus produced by infected cells in response to irritant chemicals released by the immune system.
Antibody Production
Once the immune system becomes aware of a specific invader, its response becomes more specific. White blood cells called B-lymphocytes begin to produce antibodies, explains Dr. Sherwood, which bind to viruses in the bloodstream and mark them for destruction. Other immune cells, called killer cells and macrophages, search the bloodstream for pathogens and engulf them. Even once an infection has been cleared, B-lymphocytes with the ability to make antibodies to that particular infection remain in the body, meaning that it's not possible to get the same cold virus more than once.
Treatment
While bacterial infections can be treated with antibiotics, colds, like other viruses, can't. This is because antibiotics function by attacking bacterial cells. Many of the pharmaceuticals bind to bacterial cell walls, for instance, which bacteria have but human body cells lack. Dr. Black explains that since viruses aren't truly cells and don't reproduce outside of body cells, there's no way to target them with antibiotics. Instead, thankfully, the body can clear a cold on its own, given sufficient time.
References
- "Microbiology: Principles and Explorations"; Jacquelyn Black; 2008
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- Science Daily: Cold Symptoms
