The variable region of antibodies are responsible for specificity of antibodies because this region binds the antigens. These areas have unique amino acid patters that allow for complementary binding and recognition of anitgens. 

Variable region of Antibodies and Specificity

The two 'arms' of the Y-shaped antibody molecule are known as the **Fab region**, for "fragment of antigen binding." The far end of the Fab region is the variable region, which serves as the site of antigen binding.

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B Cells are responsible for the production of antibodies, also called immunoglobulins. Antibodies are present in both the blood and tissue fluids and are involved in the body’s defense against pathogens and toxins.

Antibodies

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The constant region of an antibody molecule determines its class, or isotype. The five classes of antibodies are:
- IgG
- IgM
- IgA
- IgD
- IgE

Classes of Antibodies 

Antibodies have many functions to help facilitate an adaptive immune response:
- Antibody-dependent cell-mediated cytotoxicity (ADCC)
- Opsonization
- Neutralization 
- Agglutination
- Activation of The Complement System

Functions of Antibodies

The constant region of the antibody molecule includes the trunk of the Y and lower portion of each arm of the Y. The trunk of the Y is also called the **Fc region**, for “fragment of crystallization,” and is the site of complement factor binding and binding to phagocytic cells during antibody-mediated opsonization.

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