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Antigens

ANTIGEN: is any substance able to specifically binding to an antibody or T cell receptor. However, not every antigen is able to stimulating an immune response. When it happens, we call it an immunogen. 

IMMNOGENs: it is a molecule capable of inducing a specific immune response. Therefore, every immunogen is an antigen, but not every antigen is an immunogen. As is the case with haptens, small molecules that can become immunogenic when conjugated to a specific carrier. 

EPITOPE: it is a small part of the antigen's molecular structure that is recognized by an antibody.

Antibodies

Antibodies are glycoprotein molecules produced by plasma cells in response to an immunogen (a molecule capable of triggering a specific immune response). They specifically bind to a part of the antigen known as an antigenic determinant or epitope. All immunoglobulins have a four-chain structure as a basic unit. They are composed of two identical light chains (around 23KD) and two identical heavy chains (in the 50-70KD range) held together by disulfide bonds. Three-dimensional images of the immunoglobulin molecule show that it is not straight as represented by the diagrams in the animation above . The hinge region is the molecule's flexibility site. They also present in their structure an invariant fragment (Fc) and Fab fragments, which contain the antigen-binding site. See the schematic representation by clicking the button  in the animation.
Based on differences in amino acid sequences in the constant region of heavy chains, immunoglobulins can be divided into five different classes (or isotypes):

1. IgA – It is secreted on mucosal surfaces for defense. Ex. It is present in breast milk.
2. IgD - It works primarily as an antigen receptor on B cells and acts as an Ag receptor on naïve B lymphocytes.
3. IgE – Appears on allergic and inflammatory responses.  
4. IgG – Present in opsonization; activation of the complement system (inflammation and phagocytosis); antibody-dependent cell-mediated cytotoxicity; Feedback inhibition of B cells. It is also the only type of Ig that crosses the placental barrier.
5. IgM – Expressed on the surface of naïve B cells. Responsible for killing pathogens in the early stages of B-cell mediated immunity before there is enough IgG - complement system activation.