Immunity of the skin and mucous membranes 407 



If we examine more closely the process by which the dermis sur- 

 rounds the intruders with a fibrous capsule, we readily recognise in it 

 a reaction of the macrophages of the skin. In lupus these phagocytes 

 seize the tubercle bacilli, combining to form giant cells and giving 

 rise to an exaggerated development of the connective tissue fibres. 

 Moreover, when the skin is menaced with a microbial invasion, not 

 only the local macrophages but the leucocytes are mobilised. The [427] 

 migratory white corpuscles travel through the epidermis and the 

 connective tissue layer. In spite of the absence of a lymphatic 

 circulation in the epidermis, the leucocytes penetrate into this layer 

 "and, in a section through the normal epidermis, it is very rare not to 

 find here and there some deformed and flattened leucocyte, surprised 

 just as it was creeping between the cells of the rete mucosa or of the 

 stratum granulosum" Immediately that the epidermis or the dermis 

 finds itself menaced with a microbial invasion, an accumulation of 

 leucocytes of all kinds is produced at once ; this may remain 

 microscopic or it may assume proportions visible to the naked eye. 

 Frequently the subjacent epithelium throws off epidermic scales 

 which are filled with leucocytes ; often also the leucocytic foci in 

 the dermis become emptied, the micro-organisms being expelled 

 along with their enemies the phagocytes. 



The tissues of the skin proper defend themselves against micro- 

 organisms as well as they are able; but so soon as the danger 

 becomes serious there is sent to their succour a whole army of mobile 

 phagocytes. This example of the defence made by the cutaneous 

 investment may serve as a prototype of that of every other region 

 of the body. Alongside a local action, there is always an intervention 

 of mobile phagocytes ; but when this action becomes insufficient, 

 a much more abundant accumulation of leucocytes than is found 

 in ordinary cases is immediately produced. 



Like the skin, the mucous membranes are invested with an 

 epithelial layer, which serves as a barrier to the entry of micro- 

 organisms. But whilst the surface of the normal skin is dry or barely 

 moistened by the secretory products of the cutaneous glands, the 

 mucous membranes are always humid, a condition favourable to the 

 multiplication of micro-organisms. Hence the mucous membranes 

 which are most exposed to contact with the air and with external 

 objects, always contain a larger or smaller number of organisms, 

 amongst which the pathogenic species, notably staphylococci, pneumo- 

 cocci and streptococci, are the most common. The part played by 



