Action of Antibodies: Neutralization, Opsonization, Complement Activation and ADCC (FL-Immuno/37)

Understanding Antibody Mechanisms in Immune Response

Overview of Antibodies and Their Function

• B cells differentiate into plasma cells upon recognizing antigens, which then produce antibodies that bind to specific antigens.

• The basic structure of antibodies is a Y-shaped molecule with two antigen-binding sites; the stalk is known as the FC region.

Mechanisms of Antibody Action

Neutralization

• Antibodies neutralize pathogens by binding to them, preventing their entry into host cells—a process termed neutralization.

• Microbes or toxins cause disease by attaching to host cell surface molecules; antibodies block this binding, thus neutralizing infectivity.

• Neutralized microbes are eliminated through phagocytosis, making this mechanism effective against viruses, bacterial toxins, and venoms.

Opsonization

• Opsonization enhances phagocytosis by coating pathogens with antibodies that bind to specific antigenic determinants.

• Phagocytic cells recognize these bound antibodies via FC receptors on their surface, facilitating pathogen elimination through phagocytosis.

Complement Activation

• The complement system consists of about 20 proteins in blood plasma; antibody-coated pathogens initiate the classical pathway of complement activation.

• This pathway leads to pathogen destruction through mechanisms like membrane attack complex formation and inflammation.

Antibody Dependent Cell-Mediated Cytotoxicity (ADCC)

Understanding ADCC Mechanism

• In cases where pathogens are too large or intracellular (e.g., viruses), traditional opsonization may not suffice for elimination.

• Antibodies coat infected or target cells displaying foreign antigens on their membranes; leukocytes with cytotoxic functions recognize these antibodies via FC receptors.

Execution of ADCC

• Binding of leukocytes to antibody-coated target cells results in degranulation and release of cytotoxic chemicals that disrupt target cell membranes.

• This process effectively kills the pathogen or infected cell, illustrating how ADCC serves as a critical immune response mechanism.