Antibody Penetration: Can Antibodies Bind Intracellular Antigens?

Typically, antibodies cannot penetrate living cells to bind intracellular antigens due to size and the impermeable cell membrane. They bind surface or extracellular antigens.

Exceptions

• Receptor-mediated internalization: Some antibodies (e.g., anti-HIV) enter via specific surface receptors.
• Endocytosis/phagocytosis: Cells may internalize antibody-antigen complexes.
• Released antigens: Antibodies bind intracellular antigens released by cell death (necrosis) or exocytosis in the extracellular space.

These mechanisms are exploited in some therapies (e.g., antibody-drug conjugates).