Skin, the body’s largest organ: layers, functions, and conditions

Reviewed by Chimene Richa, MD, 

Written by Jefferson Chen, MD 

Reviewed by Chimene Richa, MD, 

Written by Jefferson Chen, MD 

last updated: Aug 30, 2021

6 min read

The skin is the largest organ in the body. More than just impressive in size, it’s incredibly important in function. What other organ can protect against infection, regulate temperature and water balance, and make us look spiffy? (The answer is none). 

Let’s take a deep dive into what makes up our skin, common diseases of the skin, and how we can best take care of it. 

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What is the skin?

The skin is essentially the wrapper for the body. Like other wrappers, it keeps what’s inside safe from the outside and retains the moisture within. The skin also functions as the way we interact with much of our surroundings. It’s covered in nerve endings that allow us to feel the textures of the world around us. These nerve endings also allow us to react to threats—pain and temperature sensors, for instance, make sure we don’t accidentally damage ourselves due to heat or cold (Yousef, 2021). 

The skin is part of a large organ system called the integumentary system. Integumentary comes from the Latin word integumentum, meaning “a covering.” Integument is a word biologists use for the natural covering of an organism, which includes skin, rind, husk, or shell. 

What are the layers of the skin?

There are two main layers of the skin: epidermis and dermis.


The outermost layer of skin is called the epidermis. It’s a layer constantly being shed and renewed. It consists primarily of keratinocytes, specialized cells whose primary function is to protect the cells underneath. 

The surface of the epidermis is made up of tightly interlocked cells. This locks out pathogens like viruses and bacteria from entering through the skin. It’s also why it’s important if you ever have a cut or a scrape to disinfect the wound and make sure it stays clean. There are also friendly microorganisms that have colonized our skin surface, which compete with pathogens for food and nutrients. Lastly, because the surface is acidified, dry, and covered with lipids, enzymes, and antimicrobial proteins, it is difficult for harmful bacteria and fungi to grow (Yousef, 2021). 

Deep in the epidermis is where melanocytes typically reside. Melanocytes are specialized cells that produce a pigment called melanin, which helps protect the rest of our body from ultraviolet (UV) radiation from sunlight. This pigment is also what gives our skin its color. 


The dermis is a deeper layer of skin that lies just underneath the epidermis. It is much thicker than the epidermis and is responsible for giving skin its flexibility. It contains high quantities of a protein called collagen, which gives it strength and elasticity. Hair follicles, blood vessels, muscles, and nerves all live in the dermis layer. It also has sebaceous glands that secrete oils to keep the skin moist (Yousef, 2021). 

Directly beneath the dermis, in the subcutaneous tissue, is where fat is stored. It's a great place to inject medicines like insulin for diabetes because of how many blood vessels run through the area.

What is the skin’s function?

You probably don’t realize how much your skin does for you. Your skin serves many functions, including (Yousef, 2021):

  • Acts as a barrier against microorganisms (germs like bacteria, fungi, etc.) and chemicals

  • Protects against UV light damage by producing melanin

  • Prevents water loss

  • Produces vitamin D from sun exposure

  • Allows for temperature balance by retaining heat to warm you or sweating to help you cool off

  • Triggers the immune system via specialized antigen-presenting cells always on the lookout for invaders

  • Houses nerve endings that provide skin sensations like touch, hot/cold, and pain

  • Grows hair

Common skin conditions

Now that you understand what the skin does, let’s go over what happens when things go wrong. Some of the most common skin conditions you might come across include rashes, eczema, bruises, acne, warts, moles, and skin cancer


We’ve all seen and felt rashes before—angry-looking, red, and sometimes itchy patches of skin that erupt over different parts of your body. Many medical conditions can cause a rash, including viral infections, bacterial skin infections like cellulitis, fungal infections, allergic reactions, or simply skin dryness and irritation. Treating a rash is directed towards the root cause, such as antibiotics in cellulitis or changing diapers frequently in a diaper rash (Santistevan, 2017). 


A type of rash, eczema, is also called atopic dermatitis and is caused by skin inflammation. It most commonly occurs as patches of itchy, thickened, and red skin. There are many different causes of eczema, including allergies, irritation, and poor circulation. Sometimes, the exact cause of eczema isn’t known (Nemeth, 2021). 


Bruising can occur on the skin after an injury that crushes the blood vessels under the skin without breaking the skin’s surface. Bruises typically fade over time, going from reddish-black to purple-blue to greenish-yellow before disappearing. It typically takes a couple of weeks for bruises to go away but might take longer, depending on the size of the injury (MedlinePlus, 2016).


Acne is a common skin condition most people know as the cause of the pimples that popped up on their faces during puberty. Scientists don’t know the exact cause of acne. Hormonal changes seem to play a large role as they can stimulate the production of oil (called sebum) from sebaceous glands in the skin. Too much sebum can clog pores and lead to acne. A bacteria called Cutibacterium acnes (formerly Propionibacterium acnes) is also known to play a role because it eats sebum and causes inflammation in the skin. 


These are small growths that occur on the outside of the skin. They’re caused by certain strains of human papillomavirus (HPV). Some high-risk HPV subtypes are sexually transmitted and can cause cervical, anal, rectal, penile, and throat cancers (Etcheverria, 2020). Most warts are not dangerous, though.


Also called nevi (singular–nevus), moles are skin lesions caused by a collection of melanocytes, the cells that produce the pigment that gives skin its color. Most moles are benign, but some with irregular borders and multiple colors can be associated with skin cancer (Bodman, 2021).

Skin cancers

Cancers of the skin are some of the most common cancers in humans. The types of skin cancers are named because of the cells that cause them. 

  • Basal cell carcinoma is the most common type and looks like a pearly white or translucent bump on the skin. It’s most common on sun-exposed areas of the skin. 

  • Squamous cell carcinoma is the second most common skin cancer and can look like a scaly patch or ulcer that won’t go away. 

  • The final type of skin cancer is called melanoma. It’s the most aggressive type of skin cancer and causes most of the deaths due to skin cancer in the United States. The most common symptom is a new or growing irregular mole. It can often spread to other organs, so early detection is key (Mattews, 2017). 

4 ways to take care of your skin

The skin does so much for the rest of the body by protecting it from pathogens and blocking ultraviolet radiation from the sun that we should try our best to keep it healthy. So how can we practice proper skincare and keep our skin happy?

1. Avoid injury to the skin 

The skin is resilient enough to heal from minor damage, but scars will form when the wound is deep enough to go into the dermis. Scar tissue is not as strong or flexible as normal skin. Hair, sweat glands, and sebaceous glands also don’t grow back in that area. And while it’s healing, the wound is susceptible to infection and water loss (Ozgok Kangal, 2021). So, try to keep your skin protected. When injuries do happen, clean them and treat them right away. 

2. Prioritize sun protection

Sun exposure can prematurely age your skin, cause liver spots or age spots, and increase your risk of skin cancers. 

The American Academy of Dermatology (AAD), a group of skin-specialists, recommends that everyone “seek shade; wear protective clothing, including a lightweight, long-sleeved shirt, pants, a wide-brimmed hat, and sunglasses; and generously apply a broad-spectrum, water-resistant sunscreen with an SPF of 30 or higher to exposed skin.” 

And for all of you beach-goers, make sure to reapply sunscreen at least once every two hours and immediately after swimming or sweating (AAD, 2019).

3. Be gentle to your skin

Adopt a healthy skincare routine. If your skin is dry, apply a lotion moisturizer. If you notice things irritating your skin, like articles of clothing or jewelry, try removing them and wearing something else. Avoid strong soaps that can dry out your skin. 

4. Live a healthy lifestyle

Smoking is well known to prematurely age skin and increases the risk of skin cancer. Smoking also impairs wound healing. Some surgeons will delay elective or cosmetic procedures until the patient stops smoking (Krutman, 2017). 

Eating a healthy diet is essential to maintaining healthy skin as well. Make sure you’re eating a well-balanced diet. Missing essential nutrients like vitamin C, for instance, can predispose you to poor wound healing. 

Finally, manage stress in your life, as this can be a trigger for acne and rashes (Pondeljak, 2020).

Seek medical advice from your healthcare provider if you have questions or concerns about your skin. They can help you figure out what’s going on, and if appropriate, refer you to an expert that can sort out the best treatment plan for you.


If you have any medical questions or concerns, please talk to your healthcare provider. The articles on Health Guide are underpinned by peer-reviewed research and information drawn from medical societies and governmental agencies. However, they are not a substitute for professional medical advice, diagnosis, or treatment.

How we reviewed this article

Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

Current version

August 30, 2021

Written by

Jefferson Chen, MD

Fact checked by

Chimene Richa, MD

About the medical reviewer

Dr. Richa is a board-certified Ophthalmologist and medical writer for Ro.