Assume that the epidermis has been scraped off by a fall (not just scratched). Also assume that the dermis is completely untouched. Does this allow pathogens to enter the body more easily, or does the dermis still provide adequate protection?
Our epidermis is the first line defence against natural infections; it is also a part of the innate immune system. This is due to
it contains a layer of dead cells that separates the living cells of the deeper layers of epidermis and dermis from the environment.
This part of the epidermis is avascular and so pathogens can not easily enter into the bloodstream through it.
It contains natural bacterial flora (like in our gut) that kills a large number of pathogenic microorganisms with their toxins.
secretion of sebaceous gland contains some antimicrobial substances.
There are lots of other protective features of epidermis.
Dermis is richly supplied by blood vessels. So even if all the layers of dermis remain intact its high vascular nature will act as an open entrance of microorganisms. By definition abrasion involves only the epidermis or some parts of the dermis but still we need to take tetanus vaccine after getting an injury contaminated with soil. In diseases like Stevens-Johnson Syndrome (where epidermis separates from dermis) or in burn patients the skin is covered with antibiotics and dressings. The patient is also given high dose of antibiotics to fight those bacteria that still entered into the bloodstream.
This answer is a reply to the question asked in the comment of my answer given on 13th July 2016. The comment was, " Nice answer, I gather that bacteria would enter more easily, what about viruses?"
In my answer on 13th July 2016, I used the term "microorganisms" - this includes both bacteria and viruses. So the answer provided there can also be applied to viruses.
To specifically discuss on the susceptibility of the host towards viral infections after damage to the epidermis (dermis is intact) the following may prove to be helpful.
Viruses has evolutionarily developed specific surface molecules that either act as a ligand or a receptor being capable of interacting with the host surface-molecules. Example - Influenza virus contains Haemagglutinin that bind to Sialic acid containing oligosaccharides found in almost every cell. Common entry route of a virus is usually determined by the surface molecules it possesses. Most common entry routes are respiratory system, gastrointestinal system, urogenital system and inoculation by bite of insects(via insect probosis). Viral infections after skin-damage are rather uncommon.
Intact skin is an effective barrier against viral infections. Dead cell layer of epidermis prevent viral growth. Viruses are non living particles outside leaving cells; so when they come in contact with the dead cellular layer of epidermis they behave in the same way as they were in outside environment. The requirement of blood or lymph supply for replication and spread of viruses meets when epidermis is removed. But in that case there occurs an inflammatory reaction causing accumulation of immune cells and antibodies. These antibodies neutralize the viral surface molecules, thus preventing viral attachment and entry into cells ( even in presence of living cells). Immune cells secret chemicals that disintegrate viral particles.
Most common mechanism of entry into the skin, for a viral pathogens are, 1. direct inoculation 2. systemic infection and 3.local spread from an internal focus
Direct inoculation is our topic. It uses two mechanisms - (i) replication in endothelial cells and then spread into the bloodstream. Entry into endothelial cells are different for enveloped and non-enveloped viruses; enveloped virus fuses with the cell membrane, while non-enveloped viruses enter by endocytosis or receptor-mediated processes. Examples include warts, milker's nodules, primary herpes simplex etc. (ii) Vectors may introduce the pathogen directly into the bloodstream
Systemic infections- Skin is infected during high viremia. Dermis is affected first followed by epidermis. Example- chicken pox.
Local spread from an internal focus- Example, recurrent herpes simplex virul infection from dorsal nerve roots to specific skin dermatomes.
For further infomations you may read 1. Fitzpatrik's dermatology in general medicine, 8th edition, 2. Kuby Immunology, 7th edition.
The dermis provides some protection, and if it is removed the danger of deep tissue infection becomes extremely high. However, the epidermis provides an additional and important layer of antimicrobial protection. That is why first and second degree burns, opened blisters, and scrapes should be kept clean and isolated from the environment with bandages, and why antimicrobial ointments are often prescribed in such cases. The epidermis is an important waterproof barrier to environmental hazards, such as germs and dehydration, and its dryness is a natural inhibitor of microbial growth. https://www.ncbi.nlm.nih.gov/pubmed/17621950 Also: http://onlinelibrary.wiley.com/doi/10.1111/j.1600-0625.2008.00786.x/abstract