CHEMOTAXIS 179 



truded from the lymphoid organs by contraction of the smooth muscles 

 (Harvey 1 ). 



As previously stated, the eosinophiles undergo a marked increase 

 during infections with various animal parasites. 



The typical macrophages, such as the endothelial cells of serous 

 cavities (Figs. 44 and 45) and the lymph-spaces, are mostly concerned in 

 the phagocytosis of other cells and inorganic material. Brodie 2 con- 

 siders the phagocytosis of leukocytes and red corpuscles by the endo- 

 thelial cells of the lymph-glands and the spleen as the normal end of 

 these cells. It is a mistake to believe, however, that they do not in- 

 gest bacteria, since endothelial cells are extremely active phagocytically 

 for bacteria, and, on the other hand, polynuclear leukocytes may be 

 observed to contain red corpuscles, especially when aided by a suitable 

 opsonin. 



CHEMOTAXIS 



An important question in the study of the phenomena of phago- 

 cytosis is the manner in which the various leukocytes and other body- 

 cells are attracted to a focus of infection and brought into contact with 

 the microparasites or other foreign substances. It must be assumed 

 that some means of communication must exist between this point and 

 the leukocytes in the circulating blood. Since there is no direct com- 

 munication by way of the nervous system or other structural route, it 

 would appear that the only mode of communication is through the body- 

 fluids. Chemical agencies, produced either directly by the bacteria or other 

 foreign substance, or indirectly by their action upon cells at the site of resi- 

 dence in the tissues, are regarded as furnishing the attractive forces that are 

 transmitted through the body-fluids and exert what has been called chemo- 

 taxis. 



The movement of a cell in response to a chemical stimulus is a phe- 

 nomenon that is displayed by almost all motile and unicellular organ- 

 isms, whether animal or vegetable, and by the leukocytes and other un- 

 fixed cells of the higher animals. As a rule, chemical stimuli serve to 

 attract cells to the site of infection, thus constituting what is known as 

 positive chemotaxis; on the other hand, the stimuli may fail to attract 

 or actually repel the cells, or be so powerful as to paralyze them en 

 route, this constituting negative chemotaxis. 



Positive Chemotaxis. That leukocytes reach the site of an infec- 

 tion because of chemical substances produced by bacteria at this point 



1 Jour. Physiol., 1906, 35, 115. 2 Jour. Anat. and Physiol., 1901, 35, 142. 



