Immunoglobulins play a key role in humoral immunity; they defend the body either through direct attack or indirect means (by complement fixation of inflammation).

Defense through Indirect Attack

In direct attack antibodies obstruct the movement of foreign agents. Binding of antibodies with the antigenic sites of the invading foreign agents either agglutinates or precipitates them and the resulting product- the antigen-antibody complex is removed by the other immune cells through phogocytosis or cytolysis. Some of the toxins released in the body get neutralized as a result of antibody binding to the toxins molecules and later removed through phagocytes.

Binding of antibody to the antigen makes them vulnerable to phagocytes and the process that makes antigen vulnerable is known as opsonization.

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The binding of antibody to the antigen may alter the surface properties of pathogen and the pathogen fails to escape phagocytosis. Binding of antigen with IgE immunoglobulins bound to the mast cells initiates hypersensitive reactions (Refer Chapter 14 for details of hypersensitivity).

Defense through Direct Attack

Immunoglobulins bound to the antigens activate the complement system, resulting in the formation of membrane associated complex (MAC) that destroys the foreign agent; added to that they initiate inflammation reactions to optimize the defense mechanism.