322 - Multicellular Animals, Especially Man 



able to leave the blood stream through the bacteria in large numbers (Fig. 17-4). Also 



capillary walls. the leucocytes tend to surround the infected 



Phagocytosis: Another Defense against region and to prevent the bacteria from 



Infection. Some leucocytes, particularly the spreading. Many white cells succumb to 



neutrophils and monocytes, carry on an ac- toxins produced by the bacteria; and these 



tive defense against pathogenic bacteria dead leucocytes make up the chief compo- 



Fig. 17-4. Leucocyte performing diapedesis, by which it actively pene- 

 trates the wall of a capillary blood vessel, and phagocytosis, by which 

 it engulfs bacteria in the tissue spaces. Semidiagrammatic. 



when these parasites manage to enter the tis- nent of pus, as it accumulates at the infec- 

 sues through an open wound. The leucocytes tion. Besides being able to digest the bacteria 

 migrate toward the wound, passing out and other foreign particles they have phago- 

 through the walls of the capillaries in the cytized, the leucocytes secrete enzymes extra- 

 infected area by a process called diapedesis cellularly. These enzymes digest away the 

 (Fig. 17-4). The white cells arrive in force at dead or dying (necrotic) tissues, especially 

 the site of infection, and begin ingesting the near the external surface of the infection. 



Table 17-2— The Kinds of Leucocytes in Man 



A. Granulocytes (polymorphonuclear leucocytes); with distinct granules in the cyto- 

 plasm; nuclei conspicuously irregular and lobose, or even subdivided; originate from 

 bone marrow: 



1. Neutrophils: granules stain with neutral dyes, or with mixtures of acid and 

 basic dyes; nucleus, with many lobes, highly motile and phagocytic; very 

 numerous, making up about 67 percent of the total white cell count. 



2. Basophils: granules stain with basic dyes (e.g., methylene blue); nucleus usually 

 with two lobes; about 0.5 percent of total whites: nonphagocytic. 



3. Eosinophils: granules stain with acid dyes (e.g., eosin); bilobed nucleus; 3 per- 

 cent of total white cell count; nonphagocytic. 



B. Agranulocytes (lymphocytes): cytoplasm, without granules; nucleus rounded; originate 



from lymphoid tissues (lymph glands, spleen, etc.): 



1. Small lynnphocytes: about 8 microns diameter; 25 percent of total whites. 



2. Large lympliocytes: about 11 microns: 3 percent of total whites. 



3. Monocytes: about 15 microns; actively amoeboid and phagocytic; 1.5 percent 

 of total whites. 



