Table 1.

Clinical and Immunohistopathological Characteristics of FFA and LPP

Diagnosis	Distinct Histopathological Characteristics1	Shared Histopathological Characteristics	Distinct Clinical Characteristics	Shared Clinical Characteristics
LPP	More severe inflammatory infiltrate and less apoptosis (Problet et al., 2006; Gálvez-Canseco et al., 2018) Concentric lamellar fibroplasia (Gálvez-Canseco et al., 2018) More basilar layer and interfollicular epidermal damage (Problet et al., 2006) Increased melanocyte counts in the upper hair follicle (Katoulis et al., 2020) DIF shows less IgM immunofluorescence and IgG, IgA, and IgM in papillary dermis (Cerqueira et al., 2016) Increased CD68+ macrophage polarization and upregulated CD163 and IL-4 (Harries et al., 2020)	Lichenoid perifollicular lymphocytic infiltration (most evident in the superior aspects of the hair follicle) (Kurzeja et al., 2021; Stefanato, 2010) Infundibular hyperkeratosis and hypergranulosis. Hair epithelium shows vacuolar degeneration, necrotic keratinocytes, and perifollicular loss of elastin fibers with fibrosis (Ma et al., 2017) Superficial pigment incontinence (Wilk et al., 2013; Bolduc et al., 2016) Follicular plugging, epidermal/dermal clefts, and sebaceous gland destruction (Kang et al., 2008) The chronic stage shows dilated blood vessels and band-like vertical scarring beneath the papillary dermis (Kang et al., 2008) The late stage shows extensive perifollicular lamellar fibrosis surrounding the infundibulum (Kang et al., 2008) Similar expression profiles of CD1a, CD3, CD4, CD8, CD68, and IDO in immunohistochemical studies (Cerqueira et al., 2016) Increased CD8+, CXCR3+, FOXP3+ T cells, and CD68+ macrophages (Harries et al., 2020) Increased total and degranulated mast cells and CD123+ dendritic cells (Harries et al., 2020) Decreased numbers of CD1a+ and CD209+ dendritic cells in the infundibulum connective tissue sheath (Harries et al., 2020)	Asymmetric multifocal involvement of scarring alopecia (Bolduc et al., 2016) Perifollicular erythema and keratotic follicular papules (Bolduc et al., 2016) Most commonly vertex and parietal scalp, but all regions can be involved (Stefanato, 2010) Association with oral, ungueal, or cutaneous lichen planus (Bolduc et al., 2016) Dermoscopy shows elongated concentric blood vessels, violaceous-blue interfollicular areas, and big irregular white dots (Bolduc et al., 2016)	Dermoscopy shows loss of follicular ostia, peripilar white scales, and peripilar erythema (Bolduc et al., 2016) Symptoms of pruritus, pain, and burning (Vañó-Galván et al., 2014)
FFA	Extension of the inflammatory infiltrate below the isthmus (Wong and Goldberg, 2017) Islands of sparing of interfollicular epidermis (Harries et al., 2013) Less prominent inflammatory infiltrate, with more numerous necrotic keratinocytes and foreign body reaction (Problet et al., 2006) More frequent terminal catagen‒telogen hairs (Gálvez-Canseco et al., 2018) DIF shows cytoid bodies of IgM in the papillary dermis and epidermal and follicular basement membrane zones (Cerqueira et al., 2016) Increased Langerhans cells in the infundibuloisthmic region compared with that in LPP (Ma et al., 2017)		Symmetric, progressive frontotemporal hairline recession in a band-like pattern above the patient’s normally pigmented and wrinkled forehead. Less frequent recession of preauricular and postauricular areas and occipital scalp. Dermoscopy shows peripilar white scales and erythema, regularly distributed red or gray dots in eyebrows, and peripilar erythema (Bolduc et al., 2016) Skin-colored facial papules. Marked or complete loss of eyebrows, typically beginning laterally (Bolduc et al., 2016) General thinning of the beard and peripheral body hair. Absence of vellus hair in the hairline.

Abbreviations: DIF, direct immunofluoresence; FFA, frontal fibrosing alopecia; LPP, lichen planopilaris.

¹Although several studies have found histopathological differences between LPP and FFA, all concluded that the findings are too subtle to distinguish the entities without clinical correlation.