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Last updated November 21, 2019. 6 minute read

Skin: the body's largest organ—everything you need to know

Our skin 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.

Dr Jefferson Chen Md Written by Jefferson Chen, MD
Reviewed by Mike Bohl, MD, MPH

The skin is the largest organ in the body. More than just prodigious 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.


  • The skin is the largest organ in the body and acts as a barrier against infection, along with many other functions.
  • The skin consists of the epidermis and dermis layers, each with its own functions. The hypodermis layer lies just beneath the skin.
  • Common skin problems include rashes, acne, moles, warts, and skin cancer.
  • Taking care of our skin involves avoiding injury, protecting it from the sun, being gentle to it, and making healthy lifestyle choices.

What is the skin, and what does it do?

The skin is essentially the wrapper for the body. Like other wrappers, it keeps what’s inside safe from 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.

The skin is part of a large organ system called the integumentary system. Integumentary comes from the integument, which is a word biologists use for the natural covering of an organism, which includes skin, rind, husk, or shell. Integument comes from the Latin integumentum, meaning ‘a covering.’ Other parts of the integumentary system include your hair, nails, and sweat glands.


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There are two main layers of the skin. The outermost layer is called the epidermis, a layer that’s 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. The surface of the epidermis is acidified, dry, and covered with lipids, enzymes, and antimicrobial proteins, making it difficult for harmful bacteria and fungi to grow. Specialized cells that are part of the immune system called antigen-presenting cells also hang out in the epidermis, on the lookout for invaders. If they find a pathogen, they can recruit an immune response to fight off the infection. There are also friendly microorganisms that have colonized our skin surface, which compete with pathogens for food and nutrients. Another role of the epidermis is producing vitamin D from sun exposure. Vitamin D is especially important for bone health. At the bottom of the epidermis are where melanocytes typically reside. They 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 deeper layer of skin is called the dermis. It lies just underneath the epidermis. The dermis 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 originate in the dermis, along with sweat glands that cool the body and sebaceous glands that secrete oils that keep the skin moist. Other functions of the dermis include housing nerve endings that provide sensation at the skin.

Sometimes, a third layer called the hypodermis thought to be part of the skin as well, though it is more accurately described as just below the skin. It’s also called subcutaneous tissue and is a place where fat is stored. Other than energy storage, it also provides insulation and cushioning to the body, and is a place where blood cells and nerves are routed through. Because of how many blood vessels run through this area, it’s a good place for medicines to be injected, such as the insulin used in diabetes.

What are common skin conditions you should know about?

Now that you understand what the skin does, let’s go over what happens when things go wrong. Here are some of the most common skin conditions that you might encounter:

  • Rashes: You all seen and felt these before—angry-looking, red, and sometimes itchy patches of skin that erupt over your body. A wide range of different 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, antibiotics in cellulitis, or changing diapers frequently in a diaper rash.
  • Eczema: A subset of rashes, eczema is also called dermatitis and is caused by inflammation of the skin. 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.
  • Bruises: 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 green-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.
  • Acne: A common skin condition most people know as the cause of the pimples that popped out on their faces with puberty. Scientists don’t know the exact cause of acne. It’s understood that hormonal changes play a large role as they can stimulate the production of oily, waxy secretions called sebum from sebaceous glands in the skin. A bacteria called Cutibacterium acnes (formerly Propionibacterium acnes) is also known to play a role because it consumes sebum and causes inflammation in the skin.
  • Warts: 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 cause cervical, anal, rectal, penile, and throat cancers.
  • Moles: Also called a 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 cancer.
  • Skin cancer: 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.

How can we take care of our skin?

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

First, we need to avoid injury to the skin. The skin is resilient enough to heal from minor damage, but when the wound is deep enough to go into the dermis, scars will form. Scar tissue is less strong and flexible compared to 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, the typical functions that the skin is typically great at.

Second, sun protection is key. 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, 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.

Third, be gentle to your skin. If your skin is dry, apply a moisturizer. If you notice things irritating your skin, like articles of clothing or jewelry, try removing it and wearing something else. Avoid strong soaps that can dry out your skin.

Finally, 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. Eating a healthy diet is important 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. And finally, manage stress in your life, which can be a trigger for acne and rashes.

Seek out your healthcare provider if you have a question or concern 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.