Your skin is your body's largest organ. It covers and protects your body. Your skin

Holds body fluids in, preventing dehydration
Keeps harmful microbes out, preventing infections
Helps you feel things like heat, cold, and pain
Keeps your body temperature even
Makes vitamin D when the sun shines on it

Anything that irritates, clogs, or inflames your skin can cause symptoms such as redness, swelling, burning, and itching. Allergies, irritants, your genetic makeup, and certain diseases and immune system problems can cause rashes, hives, and other skin conditions. Many skin problems, such as acne, also affect your appearance.

NIH: National Institute of Arthritis and Musculoskeletal and Skin Diseases