Answer
Antibodies are the effector molecules of humoral immune responses. They are synthesized by plasma cells and protect the body against harm by foreign bodies-- antigens on microbes and potentially harmful substances. Though they usually do not themselves destroy antigens, they inactivate them and mark/tag them for destruction.
The modalities of inactivation and tagging of antigens include neutralization, precipitation, agglutination, and complementation. But the event that is common in antibody- antigen reactions is the formation of antibody-antigen immune complexes.
The usual targets of antibodies are extracellular pathogens intact bacteria, free viruses and soluble foreign molecules-- pathogens that are free in body secretions, tissue fluid, and circulating blood and lymph. Normally antibodies do not invade solid issue unless there is a lesion present.
Work Step by Step
After the antibody-antigen complex is formed, the following methods are used to destroy the pathogen/antigen:
Neutralization: The Ab binds and blocks the potentially harmful site on the organism or toxin, and prevent its binding to body cells. The destruction of the complex is completed by phagocytes.
Agglutination: In the method the antibody (polymer) binds the antigen at several sites , and cross-links the complexes . The result is agglutination or clumping ( of foreign cells) that results in immobilization. IgM polymers with 10 binding sites play an important role in this process. Phagocytes finish the job of destruction. Agglutination operates in mismatched blood transfusion immune reactions.
Precipitation; This method is used for soluble antigens. Antigen-antibody complexes come out of solution and ettle. This facilitates phagocytosis
Complement fixation: This is the method of defence used against intracellular bacteria and
obsolete blood cells. In this process several antibody molecules bind on adjacent antigen sites on the same cell. Complement binding sites on the stem region of the antibodies then align ; this triggers fixation of complement into the antigenic cell surface. Complement fixation triggers cell lysis, which destroys the antigenic entity.
Complement fixation also promotes opsonization which facilitates phagocytosis.
Although antibodies can sometimes "catch" and destroy viruses as they are invading cells, in general, the humoral response is not very effective against pathogens like the tuberculosis bacilli, and viruses that invade body cells and multiply within them. These pathogens must be dealt with by the cellular arm of adaptive immunity.
