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.

Fc Region of Antibodies

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.

Fab Region of Antibodies

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.

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Components of Humoral Immunity

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An antigen is a molecular structure on the surface of a pathogen that the immune system recognizes as foreign or potentially harmful. They stimulate the production of antibodies or activate other immune cells to attack and neutralize it.

Antigens

Antibodies

B lymphocytes or B cells, mature in the bone marrow and are responsible for the production of glycoproteins called antibodies, or immunoglobulins. 

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