Antigens and Antibodies

The Nature of Antigens

Chemical substances that demonstrate immunogenicity, stimulate formation of antibodies or sensitized T cells.

Usually a foreign substance

Most are proteins or complex polysaccharides – surface irregularity makes good sites for antigen receptor interaction on the surface of B-cells and T-cells (or the antigen binding site of an antibody).

Large molecules

Antigenic determinant groups (epitopes) - the specific regions on an antigen recognized by specific antigen receptors, that is, the part of the antigen that fits into the antigen binding site of the antigen receptor.

Hapten – low molecular weight, can bind to the antigen recognition site, but needs a "carrier" (a larger molecule it can associate with) to stimulate a response.

The Nature of Antibodies

Antibodies are proteins (immunoglobulins, or, gamma globulins) produced by plasma cells in response to stimulation by a specific antigen, capable of binding to the antigen that stimulated their production.

(This is why they're referred to as gamma globulins)

Antibodies have at least two identical antigen binding (valence) sites.

Antibody Structure

Monomer - a single bivalent antibody unit

Multivalent antibodies are composed of monomers.

Monomers consists of:

• Two heavy chains
• Two light chains

Chains have:

• Variable portion - the antigen binding site
• Constant portion - forms the base and Fc (the fragment that crystalizes in cold storage) region.

The variable portion will always be the same on antibodies produced by the same plasma cell, and will always recognize the antigen that originally stimulated its production.

The structure of the Fc region determines the class of an antibody (see immunoglobulin classes, below) and can be changed by an activated B-cell (class switching) without the antigen specificity of the antibody changing.

Depending on the structure, the Fc region may bind and activate complement, and may be recognized by Fc receptors on macrophages, dendritic cells, neutrophils, eosinophils, mast cells, and NK (natural killer) cells.

Immunoglobulin Classes

• IgG – monomer, most prevalent in serum; provides naturally acquired passive immunity, neutralizes bacterial toxins, participates in complement fixation, and enhances phagocytosis.
• IgM – present on the surface of both mature and immature B-cells as a monomer and is the first secreted antibody after B-cell activation; secreted as a pentamer, involved in agglutination and complement fixation.  Initiates primary immune response (more later).
• IgA – serum IgA is a monomer, secretory is a dimer; protects mucosal surfaces.
• IgD – monomer, present on surface of mature B-cells, especially memory cells, may be important for the secondary immune response, may be important in regulating the immune response
• IgE – monomer, binds to mast cells and basophils, involved in allergic reactions.