An isotype refers to any related proteins from a particular gene family. In immunology, the "immunoglobulin isotype" refers to the phenotypic variations in the constant regions of the heavy and light chains. In humans, there are five heavy chain isotypes and two light chain isotypes:

• heavy chain 
  • α - IgA 1, 2
  • δ - IgD 
  • γ - IgG 1, 2, 3, 4
  • ε - IgE
  • μ - IgM
• light chain
  • κ
  • λ