Skin Care and Esthetics Theory: Skin Biology, Skin Typing, Lesions, and Ingredients
This domain tests whether you can explain what skin is, what it does, and how products change it. Expect questions on histology, barrier function, skin typing, primary and secondary lesions, ingredient chemistry, and the reasoning that links a client's presentation to a safe, defensible treatment plan.
Histology you must be able to reconstruct
- Epidermis, deep to superficial: basale (mitosis, melanocytes, Merkel cells), spinosum (Langerhans cells, immune surveillance), granulosum (keratohyalin, cells flattening), lucidum (thick skin of palms and soles only), corneum (15–30 layers of keratinized cells - the barrier).
- Dermis: the papillary layer carries capillaries and Meissner corpuscles; the reticular layer carries collagen, elastin, fibroblasts, sebaceous and sudoriferous glands, hair follicles, and pressure receptors.
- Subcutaneous layer: adipose cushioning, insulation, energy storage.
Barrier, pH, and water loss
The acid mantle sits near pH 4.5–5.5 and depends on sebum, sweat, and fatty acids. Barrier lipids - ceramides, cholesterol, fatty acids - form lamellar sheets in the corneum. Depleted by over-exfoliation, harsh cleansing, or retinoid overuse, they let transepidermal water loss rise, and the client reports tightness, stinging, and flaking.
Type versus condition
Types are largely genetic and oil-driven: normal, dry, oily, combination. Conditions are things the client has right now - dehydration, acne, rosacea, melasma, photoaging, sensitivity - and they change. Record both, because "oily but dehydrated" calls for a different plan than "oily."
Lesions and what they mean for you
Primary lesions arise first (macule, papule, pustule, vesicle, bulla, wheal, nodule, cyst); secondary lesions are what happens to them (scale, crust, excoriation, fissure, scar, ulcer, atrophy). Open comedones can be extracted; inflamed papules, infected pustules, and undiagnosed pigmented lesions should not be treated - refer.
Ingredients by function
Humectants (glycerin, hyaluronic acid) pull water in; emollients (esters, squalane) soften and fill cracks; occlusives (petrolatum, dimethicone) slow water loss. Keratolytics (glycolic, salicylic, lactic acids) loosen corneocytes; retinoids accelerate turnover; antioxidants (vitamin C, niacinamide) blunt oxidative stress; physical filters (zinc oxide, titanium dioxide) block UV.
Worked example
A client reports a shiny nose and forehead but "tight, flaky cheeks." Reasoning: shine means active sebum; flaking means barrier compromise, not an absence of oil. The plan is a gentle non-foaming cleanser, a humectant-plus-ceramide moisturizer, limited exfoliation, and no extraction on the flaking areas - not an oil-free mattifying regimen, which would deepen the tightness.
Traps that cost marks
Calling dehydration a skin type; confusing sebaceous with sudoriferous glands; claiming massage builds collagen; assuming Fitzpatrick type predicts sensitivity (it predicts UV response only); treating rosacea with heat, vigorous exfoliation, or extractions.