Characteristics of the Immunoglobulin (Ig) Classes

Immunoglobulin (Ig) Classes

isotypes, differences in classes due primarily to variations in the Fc fragment; classes are differentiated with shorthand names (Ig, followed by a letter: IgG, IgA, IgM, IgD, IgE)

IgA

Dimer, Monomer Number of Antigen binding sites: 4, 2 Percentage of Total Antibody Serum in serum: 13% Average Half-Life in Serum (Days): 6 Crosses Placenta: No Fixes Complement: No Fc Binds to: Biological Function: Secretory antibody; on mucous membranes

IgE

Monomer Number of Antigen binding sites: 2 Percentage of Total Antibody Serum in serum: .002% Average Half-Life in Serum (Days): 2.5 Crosses Placenta: No Fixes Complement: No Fc Binds to: Mast cells and basophils Biological Function: Antibody of allergy, worm infections

IgD

Monomer Number of Antigen binding sites: 2 Percentage of Total Antibody Serum in serum: 1% Average Half-Life in Serum (Days): 3 Crosses Placenta: No Fixes Complement: No Fc Binds to: Biological Function: Receptor on B-cells

IgG

Monomer Number of Antigen binding sites: 2 Percentage of Total Antibody Serum in serum: 80% Average Half-Life in Serum (Days): 23 Crosses Placenta: Yes Fixes Complement: Yes Fc Binds to: Phagocytes Biological Function: Long-term immunity; memory antibodies; neutralizes toxins, opsonizes, fixes complement

IgM

Pentamer Number of Antigen binding sites: 10 Percentage of Total Antibody Serum in serum: 6% Average Half-Life in Serum (Days): 5 Crosses Placenta: No Fixes Complement: Yes Fc Binds to: Biological Function: Produced at first response to antigen; can serve as B-cell receptor