THE ADAPTIVE IMMUNE SYSTEM

II. HUMORAL IMMUNITY

A. ANTIBODIES (IMMUNOGLOBULINS)

3. The 5 Classes (Isotypes) of Human Antibodies

Fundamental Statements for this Learning Object:

1. IgG makes up approximately 80% of the serum antibodies, is a monomer with 2 Fab sites. The Fc portion can activate the classical complement pathway, bind to macrophages and neutrophils to enable opsonization, bind to NK cells to promote ADCC, and can cross the placenta.
2. IgM makes up approximately 13% of the serum antibodies, is the first antibody produced during an immune response, is found mainly in the blood, and is a pentamer with 10 Fab sites. The Fc portion can activate the classical complement pathway. Monomeric forms of IgM are found on the surface of B-lymphocytes as B-cell receptors.
3. IgA makes up approximately 6% of the serum antibodies, is a dimer with 4 epitope-binding sites and is found mainly in body secretions as secretory IgA (sIgA) where it protects internal body surfaces exposed to the environment by blocking the attachment of bacteria and viruses to mucous membranes.
4. The Fc portion of secretory IgA binds to components of mucous and contributes to the ability of mucous to trap microbes, and can bind to macrophages and neutrophils to enable opsonization, and can activate the lectin complement pathway and the alternative complement pathway.
5. IgD makes up approximately 0.2% of the serum antibodies, is a monomer with 2 Fab sites, is found on the surface of B-lymphocytes as a B-cell receptor, and may play a role in eliminating B-lymphocytes generating self-reactive autoantibodies.
6. IgE makes up about 0.002% of the serum antibodies, is a monomer with 2 Fab sites, and is made in response to parasitic worms (helminths) and arthropods. It is also often made in response to allergens. The Fc portion of IgE can bind to mast cells and basophils (see Fig. 8) where it mediates many allergic reactions, and the Fc portion of IgE made against parasitic worms can bind to eosinophils enabling opsonization. IgE may also protect external mucosal surfaces by promoting inflammation.

LEARNING OBJECTIVES FOR THIS SECTION


Humoral Immunity refers to the production of antibody molecules in response to an antigen (def). These antibody molecules circulate in the blood and enter the tissue via inflammation. Humoral immunity is most effective microbes or their toxins located in the extracellular spaces of the body.

Antibodies or immunoglobulins (def) are specific glycoprotein configurations produced by B-lymphocytes and plasma cells in response to a specific antigen and capable of reacting with that antigen.

In this section we will look at the 5 classes of human antibodies.


The 5 Classes or Isotypes of Human Antibodies (Immunoglobulins (def))

There are 5 classes or isotypes of human antibodies:

a. IgG (Immunoglobulin G; 4 subclasses, IgG1-4)

 

b. IgM (Immunoglobulin M)

c. IgA (Immunoglobulin A; 2 subclasses, IgA1-2)

d. IgD: (Immunoglobulin D)

e. IgE (Immunoglobulin E)

Each day an average adult produces approximately three grams of antibodies, about two-thirds of this IgA.

 


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Updated: Aug., 2014
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