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The skin is the largest organ by surface area and weight, with three main layers: epidermis, dermis, and subcutaneous.

The stratum basale (basal layer) is the deepest epidermal layer where new keratinocytes are produced and where melanocytes reside.

The stratum lucidum is a clear, thin layer found only in the thick skin of the palms and soles.

Melanocytes are located in the stratum basale and produce melanin, which determines skin color and helps shield deeper cells from UV light.

The stratum corneum is the outermost layer; its dead, flattened keratinocytes are what cosmetic exfoliation targets.

The dermis is the middle layer of skin and contains collagen, elastin, blood vessels, nerves, hair follicles, and sebaceous and sweat glands.

The subcutaneous (subcutis) layer is mostly adipose (fat) and connective tissue that cushions and insulates the body.

Licensee exfoliation is limited to the dead cells of the stratum corneum; removal of living layers is the practice of medicine.

Sebaceous glands secrete sebum, the oily substance that lubricates skin and hair; sudoriferous glands produce sweat.

The cortex is the middle layer; it holds melanin pigment and most of the protein bonds that determine hair strength and elasticity.

The dermal papilla is a small mound of connective tissue at the base of the follicle that supplies blood and nutrients to the growing hair.

The cuticle is the outermost layer of the hair; its overlapping scales protect the cortex and must be opened for chemical services.

Anagen is the active growth phase. On the scalp, it lasts 2–7 years and accounts for the majority of hairs at any time.

Catagen is the short transition phase between active growth and resting; it lasts roughly 1–2 weeks.

Telogen is the resting phase that lasts about 3 months; at the end of telogen the hair sheds and the follicle re-enters anagen.

Daily loss of about 50–100 telogen hairs is a normal part of the hair cycle. Sudden, heavy, or patchy loss warrants medical referral.

The matrix, hidden under the proximal nail fold, is the living tissue that produces the nail plate. Damage to the matrix can cause permanent deformity.

The lunula is the visible portion of the matrix and appears as a whitish half-moon at the proximal end of the nail plate.

The hyponychium is the skin seal beneath the free edge that helps prevent microorganisms from entering the nail bed.

Pus and swelling suggest paronychia (an infection). Service must be declined and the client referred; the salon is not a treatment site for infections.

Staphylococci are cluster-forming cocci. Streptococci grow in chains. Bacilli are rod-shaped; spirilla are spiral.

Streptococci grow in chains and can cause strep throat, impetigo, and other skin infections.

Bacilli are rod-shaped bacteria. They include the agents of tuberculosis (Mycobacterium tuberculosis) and tetanus (Clostridium tetani).

Spiral (spirilla / spirochete) bacteria cause syphilis and Lyme disease. Colds and flu are viral; athlete's foot is fungal.

Only a minority of bacteria cause disease; most are harmless or beneficial. Viruses are much smaller than bacteria; fungi are a separate kingdom.

Both hepatitis B and HIV are bloodborne pathogens transmitted through blood and certain body fluids — a key reason implements that may contact blood must be properly disinfected.

HPV causes warts, including plantar and common warts. Warts are contagious; service should be modified to avoid the area, and clients referred to a physician.

California requires hospital-grade, EPA-registered disinfectants that are bactericidal, virucidal, and fungicidal for multi-use implements that contact skin.

Despite its name, ringworm is a fungal infection (dermatophyte). It is contagious; service should be declined and the client referred to a physician.

Head lice are parasitic insects that feed on scalp blood. They are spread by direct contact and through shared combs, brushes, and headwear.

Active infestation must not be treated in the salon; service is declined and the client is referred. Tools used must then be disinfected per California rules.

Onychomycosis is a fungal infection of the nail plate/bed, often causing thickening, discoloration, and crumbling. It is contagious; service is declined.

Onycholysis is the painless separation of the nail plate from the bed. Causes include trauma, allergic reaction, and certain diseases. Enhancements should not be applied over a separated plate.

Paronychia is an infection (often bacterial, sometimes fungal) of the skin and nail folds around the nail. It presents with redness, swelling, and often pus.

Signs strongly suggest onychomycosis, a fungal disease. Treating or 'fixing' diseased nails is outside the scope of practice and risks spreading infection.

Open, weeping lesions are signs of possible contagious infection. Estheticians do not diagnose or treat infections; service is declined and the client referred.

Acne involves blockage and inflammation of the pilosebaceous unit. Estheticians can extract non-inflamed comedones; medical treatment of severe acne is outside scope.

Psoriasis is a chronic autoimmune skin disorder characterized by silvery scales. Cosmetologists adapt the service to avoid irritation but do not treat it.

Androgenetic alopecia is pattern hair loss linked to genetics and hormones. Alopecia areata causes round patches; tinea capitis is scalp ringworm.

Alopecia areata is an autoimmune condition causing sudden round patches of hair loss with smooth, non-scarred scalp. It warrants medical referral.

Common dandruff involves accelerated shedding of stratum corneum cells from the scalp. It is not contagious and is generally managed with anti-dandruff products.

Cranial nerve VII (the facial nerve) controls the muscles of facial expression. Cranial nerve V (trigeminal) is the chief sensory nerve of the face.

Cranial nerve V (trigeminal) carries most sensation from the face. It has three main branches: ophthalmic, maxillary, and mandibular.

The orbicularis oculi circles each eye and closes the eyelid. The frontalis raises the eyebrows; the masseter chews; the platysma is a broad neck muscle.

The frontalis (front portion of the occipitofrontalis) lifts the eyebrows and wrinkles the forehead horizontally.

The masseter is a powerful muscle of mastication that raises the mandible to bring the teeth together.

The parietal bones form the sides and top of the cranium. The maxilla and mandible are facial/jaw bones; the clavicle is in the shoulder.

The radial nerve supplies the back of the arm and the thumb side of the hand. Median and ulnar nerves cover the rest of the hand.

Flexor muscles in the forearm bend the wrist and fingers toward the palm. Extensors straighten them away from the palm.

Licensee massage is relaxation-oriented and surface level. Treating joints, diagnosing conditions, or providing therapeutic medical massage is outside scope.