Structural Basis for the Differences between Type I and Type II Human γ-Globulin Molecules

Polypeptide chains common to all γ-globulin groups (i.e., 6.6 S γ-globulins, β_2A-globulins, γ₁-macroglobulins, and γ-microglobulins) were found in two forms (I and II) differing in antigenic composition. The two types of common polypeptides were characterized as “L” chains with a molecular weight of about 20,000.

Each 160,000 molecular weight human γ-globulin molecule appears to be composed of two “L” polypeptide chains with identical (type I or II) antigenic properties (which account for 25% of the molecule) and class specific (γ, β_2A or γ_1M-type) “H” components (which account for the remaining 75% of the molecule).