6oo TROPICAL HYGIENE, SANITATION, ETC. 



The Protection afforded by these sera and vaccines is accounted for 

 along the lines of live principal hypotheses as follows : — 



(i) The Exhaiislion (Pabulum) Theory. 



During the first attack the invading organisms remove certain 

 chemical substances necessary for their own growth and in consequence 

 the same species cannot later reinvade the tissues with success owing 

 to shortage of essential chemical food. This view is not widely held. 



If correct, each specific disease must have a specific chemical 

 pabulum as one disease is only protective against itself. 



(2) The Antidote or Retention Theory. 



It is assumed that after a bacterial atack some product is left behind 

 which inhibits any further multiplication of the organism. This 

 theory preceded the antitoxin and antibacterial theories. There is 

 experimental evidence in support of it. Some of the bodies being 

 formed are known as Agglutinins, which agglutinate bacteria or 

 corpuscles, Cytolysins which destroy living cells and Precipitins which 

 precipitate the albumin of the serum. 



They have been summed up as follows : — 



Antibody Antigen Action 



Antitoxin Toxin Neutralization 



Precipitin Coagulable protein Precipitation 



Agglutinin Cells, bacteria, &c. Clumping 



Cytolysin, including bacte- Cells, bacteria, &c. Prepares cells, bacteria, &c., 



riolysin and haeniolysin for solution by complement 



Opsonin Cells, bacteria, &c. Prepares cells, bacteria, &c., 



for ingestion by leucocytes 



Some authorities stale that opsonin should not be classed as an 

 antibody. Possibly all antibodies may play the part of an opsonin. 



(3) The Acclimatisation Theory. 



This is a condition of increased cellular resistance following a 

 bacterial attack. The cells acquire a tolerance of the poison so that 

 ordinary symptoms do not manifest themselves. 



(4) The Phagocytic Theory. 



The phagocytic cells, polymorphonuclear leucocytes, mono- 

 nuclears, &c., absorb and destroy the invading bacteria. The 

 inoculation of an attenuated virus is said to stimulate the cellular 

 activity in this direction. Metchnikoff's views were based on this 

 theory. 



Opsonin (opsono = I cater for) is a chemical substance in the blood 

 stimulating the leucocytes to ingest bacteria. The extent of its action, 

 known as the opsonic index, is a valuable test in estimating the trend 

 of immunization. This opsonic index is estimated by comparing th? 



number of bacteria ingested by a given number of leucocytes in a 



normal serum and in a " stimulated " serum. 



