1. A method of diagnosing and treating meibomian gland dysfunction (MGD) in a subject, the method comprising: (a) determining in an eyelid of a subject one or more of: (i) a number and/or density of immune cells present within one or more ducts/ductules in one or more meibomian gland(s), (ii) a level of glandular/ductal obstruction in one or more meibomian gland(s), and (iii) a size of one or more ducts/ductules present in one or more meibomian gland(s); (b) comparing the one or more of: (i) the number and/or density of immune cells present within one or more ducts/ductules in one or more meibomian gland(s), (ii) the level of glandular/ductal obstruction in one or more meibomian gland(s), and (iii) the size of one or more ducts/ductules present in one or more meibomian gland(s), determined in the eyelid of the subject to one or more corresponding reference values; (c) identifying a subject having in an eyelid one or more of: (i) an elevation in the number and/or density of immune cells present within one or more ducts/ductules in one or more meibomian gland(s), (ii) an elevation in the level of glandular/ductal obstruction in one or more meibomian gland(s), and (iii) an elevation in the size of one or more ducts/ductules present in one or more meibomian gland(s), compared to the one or more corresponding reference values, as having MGD; (d) determining the number and/or density of immune cells in a palpebral conjunctival epithelium, and/or a number and/or density of immune cells in a palpebral conjunctival substantia propria in the eyelid of the subject; (e) comparing the number and/or density of immune cells in the palpebral conjunctival epithelium, and/or the number and/or density of immune cells in the palpebral conjunctival substantia propria determined in the eyelid of the subject, to one or more corresponding reference values; (f) further identifying a subject having in an eyelid one or more of an elevation in the number and/or density of immune cells in the palpebral conjunctival epithelium, and an elevation in the number and/or density of immune cells in the palpebral conjunctival substantia propria, compared to the one or more corresponding reference values, as having MGD; and (g) selectively orally or topically administering to a subject identified as having MGD and determined to have an elevated number and/or density of immune cells in the palpebral conjunctival substantia propria as compared to a reference level, at least one anti-inflammatory antimicrobial agent, and/or selectively performing meibomian gland probing on a subject identified as having MGD and determined to have an elevation in the level of glandular/ductal obstruction in one or more meibomian gland(s) compared to a reference level.