*2.2.1 Ceramides*

Ceramides, alongside cholesterol and free fatty acids, are the predominant components of the SC and comprise 30–40% of the SC lipid matrix by mass. They are composed of long chain sphingoid bases (e.g., sphingosine) which are linked to long chain free fatty acids. Incorporating the skin's naturally occurring ceramides such as ceramide I (ceramide EOP) and ceramide III (ceramide NP) in topical moisturisers can help to improve both healthy and psoriatic skin by replacing decreased or even depleted ceramide levels [76]. A functional SC plays an indispensable role in ensuring the skin's flexibility and structural integrity. The ordered alignment and organisation of the lipid bilayers within the SC forms a closed system to prevent TEWL in psoriatic plaques and makes the SC more impermeable. Therefore, even a subtle change or disturbance in the amount, physicochemical characteristics and organisation of the SC ceramides can potentially initiate and/or exacerbate psoriasis [71, 77].
