Comprised of glycoproteins, antibodies in blood and body fluids belong to the humoral immune system. Sometimes referred to as immunoglobulins (Igs), antibodies have two separate forms. The body secretes these proteins in fluids or they may attach to B cells. These immune system components identify, mark, or destroy foreign substances perceived as a threat to the body. Mammals typically have five distinct types of antibodies, and each has a specific purpose.
The immune system responds to allergens, bacteria, and fungi as well as to cancers and viruses. Antibodies form from certain white blood cells. In blood and other fluids, the structure of antibodies typically resembles a Y, including two heavy curved chains and two light straight chains. The tips of the structure contain lock type openings known as paratopes that enable insertion from the key type, or eptiope end, of antigens.
Cells generally secrete the soluble form of antibodies. The body releases these antibodies into blood, fluids, and tissues throughout the body. Similar to a security system, these soluble substances constantly survey the system for invaders. The other form of antibody forms a membrane that can attach to B cells, acting as cell activators or enabling the B cell to remember certain substances. Attached to B cells, these antibodies may also interact with helper T cells to produce a fully activated cellular immune response.
The five types of antibodies are generally classified by the heavy chain. They are categorized as IgA, IgD, or IgE, along with IgG and IgM. The IgG are the smallest but most common — 75% to 80% percent — of the antibodies found in the body. Body fluids typically contain IgG isotypes.
The IgG antibodies are the only type of antibody that can cross the placenta and provide protection to an unborn child. IgA antibodies protect the body from harboring substances acquired from outside of the body and comprise 10% to 15% of immunoglobulins. IgM comprises 5% to 10% of the antibodies and is located in blood and lymph fluids throughout the body.
IgM antibodies are the first responders to infection and signal other immune cells to respond. IgE antibodies are located in the lungs, mucous membranes, and on the skin. They may respond to infections but are more commonly associated with triggering an allergic response. IgD is typically found on B cells that have never been exposed to an antigen. They generally trigger an immune response from other cells against infection.