An animal model has been used to address the question of the biological importance of the known structural difference between the two isotypes of human C4, i.e., C4A and C4B. Guinea pigs deficient in C4 were reconstituted transiently with either human C4A or C4B protein and immunized with the bacteriophage phi X174. Results from this study showed that C4A-reconstituted animals made a secondary response, i.e., switch from IgM to IgG; whereas the C4B-reconstituted animals did not.

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