68 
PRINCIPLES OF BACTERIOLOGY 
bodies formed against the endotoxin-producing bac¬ 
teria were the so-called opsonins; it is now well to em¬ 
phasize their importance in connection with phagocyto¬ 
sis ; the reason that not all bacteria are destroyed by 
phagocytosis is twofold: in some diseases, such as 
pneumonia, meningitis, etc., the number of leucocytes 
is increased from the normal 7,500 to 15,000, 20,000, 
and upwards per cubic millimeter; this is called 
leucocytosis; in some diseases there is a diminution of 
leucocytes, and this is called leucopenia; thus, first of 
all, for a successful phagocytosis there must be a 
Fig. 9.—Ehrlich’s theory of immunity, a, body cell; b, the receptor, or 
immune body; c and d bacterial poison; c, haptophore (anchoring) group; 
d, toxophore (poisonous) group. 
leucocytosis; but even that alone is insufficient, as bac¬ 
teria may repel the leucocytes; this is called negative 
chemotaxis (chemical attraction); this is the reason 
opsonins are so important because they help attract the 
leucocytes (i. e., exert positive chemotaxis) and render 
bacteria more easily digestible by leucocytes. 
2. Ehrlich’s Humoral Theory. —According to Ehr¬ 
lich every body cell consists of a central part which is 
concerned in its function (that is, if it is a muscle cell 
its function is contractile, if it is a gland cell its func¬ 
tion is to secrete, etc.), and another part which is con- 
