TABLE 1.
Human isotype classes and associated Fc-binding proteins
| Human isotype classes | IgAa | IgE | IgGa | IgM | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Fc binding protein | |||||||||||
| Receptors | Polymeric Ig | Fc α | FcRn | Fc ε | Fc γ | FcRn | Polymeric Ig | Fc α/μ | |||
| Complement component | C1q | C1q | C1q | ||||||||
| Effector functionsb | |||||||||||
| Complement-dependent cytotoxicity (CDC) | Somewhat | Very well | Exceedingly well | ||||||||
| Antibody-dependent cell-mediated phagocytosis (ADCP) | Somewhat | Yes | Likely | ||||||||
| Antibody-dependent exocytosis of lysosomes to destroy large parasites | Yes | Yes | |||||||||
| Antibody-dependent cell-mediated cytotoxicity (ADCC) | Somewhat | Yes | |||||||||
| Stimulate secretion of inflammatory mediators | Extremely well | Somewhat | |||||||||
| Antibody relocation | |||||||||||
| Transcytosis across: | Epithelium into mucosal surfaces of the intestinal and respiratory tracts (dimeric IgA only) | Epithelium into mucosal surfaces of the intestinal and respiratory tract and glandular secretions such as breast milk, saliva, and tears | Epithelium into mucosal surfaces of the intestinal and respiratory tracts (and reverse transcytosis back to the underlying tissue); | Epithelium into mucosal surfaces of the intestinal and respiratory tracts | |||||||
| Placenta into the blood stream of the developing fetus in utero | |||||||||||
| Maintain blood IgG levels | Recycles antibody to prevent degradation and excretion from the body | ||||||||||
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aOverall ability varies between subclasses.
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bStimulated following engagement of antigen–antibody-binding protein.
