MESENCHYMAL CELLS 



finds no evidence that they can be derived directly from 

 lymphocytes. They can, however, be derived from forms 

 morphologically identical with those from which lympho- 

 cytes are derived. 



In chronic conditions associated with hypersensitivity, there 

 is usually evidence of a modified macrophage response in the 

 form of an aggregation of metalophil cells (Marshall, 1956). 

 In long-continued immunization of rabbits, the Malpighian 

 bodies in the spleen may show a perifollicular accumulation 

 of such cells, and in most chronic hyperplasias of lymph nodes 

 associated with infection there is proliferation of the macro- 

 phages lining the sinuses. 



The epithelioid cells of the tubercle are metalophil and 

 are probably derived from blood monocytes rather than 

 lymphocytes. Marshall considers the possibility that these 

 metalophil reactions are indicative of immunological hyper- 

 sensitivity but, while not completely rejecting it, considers 

 that the more likely interpretation is that they are a response 

 to lipid material either provided by an infecting micro- 

 organism or by some pathological process in the body. 



The essence of the sarcoid lesion (and the Kveims reaction) 

 is the formation of an epithelioid (metalophil) nodule. If 

 this, as we have claimed, is an immunological response, we 

 shall have to include, as members of the immunologically 

 characterized clones, macrophages as well as lymphocytes 

 and plasma cells. Depending on how new experimental 

 approaches develop, it may become possible to decide 

 whether their membership of a responsive clone is by virtue 

 of direct descent or by some process of transfer from cells of 

 the lymphocyte-plasmacyte series. 



(v) Fibroblasts can arise from any of the cells of the other 

 types, but it is uncertain whether they retain any poten- 

 tiality of reversion to primitive types and redifferentiation 

 in another direction. 



116 



