The cells of the agnathan AIS have roles roughly equivalent to those of B-cells and T-cells, with three lymphocyte lineages identified so far: VLRA and VLRB were identified in 2009,[1] while VLRC was discovered in 2013.
Jawless vertebrates do not have immunoglobulins (Igs), the key proteins to B-cells and T-cells.
However, they do possess a system of leucine-rich repeat (LRR) proteins that make up variable lymphocyte receptors (VLRs).
VLRA+ LLCs were unable to bind Bacillus anthracis, Escherichia coli, Salmonella typhimurium, or Streptococcus pneumoniae before or after immunization, suggesting that VLRAs require antigen processing like TCRs.
[1] However, MHCs or MHC-like molecules that could present processed antigens have not been found in lampreys,[8] and some VLRAs expressed in yeast were able to directly bind to antigens.