What is the Basic Antibody Structure?

Mark Wollacott

The basic antibody structure is a Y-shaped protein molecule featuring two heavy and two light polypeptide chains. One can visualize the Y antibody structure broken down as a V standing on a lowercase L with a line on either outside edge of and parallel to the V. The lowercase L, known as the Fc region of the antibody, includes the two heavy polypeptide chains, which climb upward to form the V, or the Fab region. The inside lines of the V are the ends of the heavy chains, while the outer lines are the light polypeptide chains.

Antibodies are proteins that exist in bodily fluids, such as blood.
Antibodies are proteins that exist in bodily fluids, such as blood.

An antibody, or immunoglobulin, is a protein produced by plasma cells in the body. The body’s immune system uses antibodies to recognize antigens found in hostile foreign objects, such as bacteria and viruses, and get rid of them. Each antibody is produced in reaction to a specific antigen found on the foreign invaders.

IgA antibodies can be found in the eyes and nose.
IgA antibodies can be found in the eyes and nose.

As for the antibody structure, the top ends of both sets of chains in the Fab region are known as the antigen binding site. These binding sites are the area of greatest variation between any two types of antibody. This is because the antibody will use the binding sites to attach itself to the antigen it was designed to target.

IgE antibodies are located in the lungs.
IgE antibodies are located in the lungs.

The ends of the light chains can be classed as either kappa or lambda in mammals, while lower vertebrates also have an iota form. The heavy chain make-up determines the antibody’s subclass. These heavy chains can vary in size and composition. Some are composed of around 450 amino acids while others have around 550.

IgA is found in tears.
IgA is found in tears.

The tip of each type of antibody is made up of around 110 to 130 amino acids. These tips are subdivided into two regions. The hypervariable (HV) region contains the widest variation in amino acids, while the framework (FR) region is more constant and stable. The HV region makes direct contact with the antigen. This is why it is sometimes referred to as the complimentary determining region (CDR).

The five antibody isotypes are known as IgA, IgE, IgD, IgG and IgM.
The five antibody isotypes are known as IgA, IgE, IgD, IgG and IgM.

While the top end of the antibody structure binds to the antigen, the Fc region, also known as the fragment crystallizable region, determines how the antibody deals with the antigen. This means the antibody can regulate and stimulate an appropriate immune response. The constant regions can be divided into five isotype classes: Immunoglobulin M (IgM), Immunoglobulin G (IgG), Immunoglobulin E (IgE), Immunoglobulin D (IgD) and Immunoglobulin A (IgA). The constant region composition of each isotype is identical.

Antibodies are Y-shaped and contain a region on the ends known as a paratope, which allows binding with  particular antigens.
Antibodies are Y-shaped and contain a region on the ends known as a paratope, which allows binding with particular antigens.

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