IMMUNOLOGY AND SELF-SAVING CATALYSTS 143 



or of modified antigens (e.g., diphtheria toxoid). Vaccination 

 (from vacca, a cow) infects one with the usually very mild cowpox, 

 and thereby establishes an active immunity against smallpox. 



While most antigens are proteins with large colloidal molecules, 

 certain carbohydrate and lipo-carbohydrate substances have been 

 recently shown to act as antigens. Simpler proteins do not serve 

 as antigens, for example, protamines consisting mainly of com- 

 plexes of diamino acids, and gelatin lacking tryptophane which 

 contains a benzene ring. 



Toxins are poisonous antigens. Many antigens, such as egg 

 albumin, casein, and animal sera, are not toxic in the small quanti- 

 ties needed for immunization. As Prof. J. J. Abel pointed out, 

 the Greek word toxikon originally meant "of or belonging to the 

 bow," a meaning that still survives in the word toxophilite, a lover 

 of the bow or archery. The word was carried over to the arrows 

 shot from the bow, and then to the poison into which the arrow 

 heads were usually dipped. Many substances like phenol, arsenic, 

 prussic acid and morphine which do not elicit the formation of 

 antibodies, should be called poisons, not toxins. 



Many toxins are formed by pathogenic microorganisms. Endo- 

 toxins come from the bacterial body itself on breakdown 

 (typhoid bacillus), while exotoxins are substances formed by and 

 excreted by the bacteria (e.g., diphtheria). Snake venoms and 

 the highly poisonous toxalbumins, such as abrin (from the 

 jequirity bean) and ricin (from the castor bean) induce the forma- 

 tion of specific antibodies. Some toxins, when altered chemically 

 or physically, become non-toxic but still preserve the power to 

 form antibodies to the original toxin, for example, diphtheria 

 toxin treated with formaldehyde and alum. A modified toxin of 

 this sort is known as a toxoid, and its immunizing power may well 

 be due to the slow liberation of the toxin or a closely similar sub- 

 stance. 



Synthetic Antigens 



This interesting field was developed by Dr. Karl Landsteiner 

 (Nobel prize, 1930), who coupled organic substances of known 

 chemical constitution to proteins by a process known as diazotiza- 

 tion. For example, when metanilic acid (/?t-aminobenzene sulfonic 

 acid) is treated with nitrous acid, the amino group yields a highly 

 reactive diazo group, which chemically fastens the metanilic acid 

 molecule residue to the protein. This is a counterpart of the 



