CHAPTER XIII

IMMUNITY OF THE SKIN AND MUCOUS MEMBRANES

Protective function of the skin.—Exfoliation of the epidermis as a means of ridding the animal of micro-organisms.—Localisation and arrest of micro-organisms in the dermis.—Intervention of phagocytes in the defence of the skin.

Elimination of micro-organisms by the conjunctiva.—Microbicidal function of the tears.—Absorption of toxins by the conjunctiva.—Protection of the cornea.—Elimination of micro-organisms by the nasal mucosa.—Protection by the respiratory channels.—Dust cells.—Absorption of poisons by the respiratory channels.

Alleged microbicidal property of the saliva.—Part played by microbial products in the protection of the buccal cavity.—Antitoxic function of the saliva.

Antiseptic action of the gastric juice.—Antitoxic function of pepsin.

Protective function of the alimentary canal.—Absence of microbicidal power from the intestinal ferments.—Protective function of the bile.—Antitoxic rôle of the digestive ferments.—Favouring and retarding functions of the intestinal micro-organisms.—Destruction of toxins by these micro-organisms.

Defensive rôle of the liver. Protective function of the lymphoid organs of the alimentary canal.

Protective function of the mucous membrane of the genital organs.—Autopurification of the vagina.

[Sidenote: [423]]

In the preceding chapters the phenomena of immunity which are exhibited within the animal body in which the portals were open for the penetration of the micro-organisms and their poisons have been studied. We had to deal almost exclusively with experimental immunity, the study of which constitutes the basis of our present knowledge concerning the general problem of immunity. In natural immunity, however, things do not follow the same course. The micro-organisms and their toxins are not introduced directly into the tissues and blood by means of a syringe or other instrument. The micro-organisms have to make their own way through the skin and the mucosae, tissues which offer a resistance more or less serious and effective; or they may have to take up their abode in the cavities of the animal organism, in order that they may be able to inundate it with their poisons. We must here review briefly these natural barriers to microbial invasion.

[Sidenote: [424]]

The skin constitutes a protective covering of great importance in connection with the preservation against microbial invasion of the delicate parts of an animal. In many of the lower and higher animals, and even in man himself, the skin becomes the seat of a microbial flora, often very abundant, in which may be found, in addition to certain inoffensive organisms, other minute parasites more or less harmful. The pyogenic cocci, staphylococci and streptococci, are constantly found on the human skin, most frequently hidden in the depths of the canals of the hair follicles. These micro-organisms seize every favourable opportunity to attack the organism, producing such local lesions of the skin as acne, pimples, boils, and erysipelas, or even becoming generalised in the blood and tissues, as in the septicaemias and pyaemias. To the skin, therefore, must be assigned a very important function in the prevention of the invasion of micro-organisms which are found constantly on the surface of the body or which, along with all kinds of dirt, are brought there accidentally.

[Sidenote: [425]]

The skin is able to fulfil this protective function from the fact that, in most animals, it is covered with a not very permeable layer of some considerable thickness. In the majority of the Invertebrata, of all classes, the surface of the body is clothed with a chitinous layer, sometimes very thin and capable of folding and following all the movements of the body; or again it may be impregnated with calcareous salts and very hard, as in the case of the integument of Insects and Crustacea, and the shell of the Mollusca. In all cases this cutaneous sheath constitutes a formidable obstacle to the entry of micro-organisms. Even in animals of very small size the thin cuticle is effective in preventing any invasion by these parasites. Thus the _Saprolegniae_, fungi so fatal to many aquatic animals, are often quite unable to pass through this cuticular layer. In order to pass this obstacle their germs must take advantage of some fissure or wound, produced by other causes. _Daphniae_, too, may often be observed to succeed in ridding themselves of the _Monospora_ with its needle-like spores by means of a mechanism which we have already described in