The single exception to the rule of class restriction of S.Ig in B lymphoproliferative
disorders was the patient in whom some lymphocytes carried
p., 7, and A chains. This patient showed also a biclonal proliferation (see
below) with either p.A or A markers. Since normal lymphocytes may carry
more than one heavy-chain class in the early stages of the immune response,
whereas class restriction occurs in the later steps,32 this double producer
clone may well represent the proliferating clone from which the single
producer clones originated.