This protein serves as a carrier for AIM in the blood to protect it from renal excretion, and it unbinds AIM to activate AIM’s apoptotic clearance function. Overproduction of this protein associated with AID results in susceptibility to encapsulated bacteria, while Pneumocystis and Cryptosporidium infections in the first year of life signal an X-linked CD40L-associated overproduction of it. Unlike a smaller counterpart, this protein’s polymerization does not depend strongly on the presence of the J-chain. This protein’s (10[1])bound conformation is C1q’s most efficient binding partner to trigger opsonization in the classical complement pathway. This protein is the larger of the two expressed by naive B cells before class-switching, and its high avidity makes it useful for responding to multivalent antigens. For 10 points, the first antibody to respond to an antigen challenge is what pentameric isotype? ■END■