Why is the left shift name given when band neutrophils increased in the CBC blood test?

 The doctrine about nuclear shift of neutrophils was proposed by (Arneth, 1905) on the basis of studies of blood at various infections and, 

especially at TB.

After complex calculations, Arneth discovered a pattern in the nucleus of neutrophils. 

Blood of a healthy individual contains:

– 5% of neutrophils with Non-segmented 

nucleus (Class I);

– 35% neutrophils with nucleus composed 

of 2 connected segments (Class II);

– 41% neutrophils with nucleus composed 

of 3 connected segments (Class III);

– 17% neutrophils with nucleus composed 

of 4 connected segments (Class IV);

– 2% neutrophils with nucleus composed of 

5 connected segments (Class VI).

During infection, the number of multisegmented neutrophils decreases. The proportion of 2-3 segmented nuclei and non-segmented nuclei (being relatively young cells) increases. According to Arneth’s scheme, the number of non-segmented neutrophils is written 

on the left (class I), then class II and then class 

III neutrophils on the right, etc. 

Hence, an increase in non-segmented and 2-3 segment nuclei will cause an increase in the number of cells located in the scheme on the left part and making a shift to the left .