What Determines Antibody Class?

Commonly referred to as the "adaptive immune response," the protection antibodies provide against foreign microbes is more important than any other branch of the human immune system. Antibodies are the only molecules capable of binding to -- and ultimately eliminating -- microbes that produce complex capsules, which normally protect them from other cells in your immune system. The different classes of antibodies are able to recognize and distinguish the components (antigens) of these capsules, which include lipids, proteins and sugar molecules. Antibodies differ in structure and function and are categorized into several classes that commonly undergo a process known as "class switching" during the course of an infection.

Naive B-Lymphocytes

• Antibodies are produced by specialized immune cells known as B-lymphocytes. These cells initially take up residence within islands of lymphoid tissue throughout the body, at which point they are referred to as "naïve B-lymphocytes". Naïve B-lymphocytes exclusively express two classes of antibodies: IgM and IgD, short for immunoglobulin class M and D, respectively. These membrane-bound antibodies act as antigen receptors for the naïve B-lymphocytes.

B-Lymphocyte Activation

• Prior to activation, naïve B-lymphocytes remain dormant within the spleen, lymph nodes and other lymphatic tissues. This strategic positioning allows them to bathe themselves in the lymphatic drainage, which constantly filters into the lymphoid tissue from the periphery. The immune system is organized in this fashion to increase the concentration of free-floating antigens in contact with stationary B-lymphocytes, thus increasing their probability of antigen recognition and subsequent activation. The first signal required for lymphocyte activation occurs when antigens come in contact with the IgM and IgD receptors on naïve B-lymphocytes.

Antibody Structure

• Despite significant variation in the antigens recognized by different classes of immunoglobulins, all antibodies share two basic features: a constant region and a variable region. The constant region, similar in all antibodies, is composed of a molecule known as the "heavy chain," which takes the shape of a Y. The type of heavy chain determines the antibody class of the immunoglobulin. The variable region, superimposed on the heavy chain, is shaped like a V shape and known as the "light chain." The sequence of the variable region determines to which antigens the antibody binds.

Function

• Different classes of antibodies carry out specific immune functions and their production varies accordingly. For instance, IgG, the most common antibody in circulation, is particularly adept at attacking extracellular viruses and tagging pathogens for elimination by phagocytes: immune cells capable of engulfing and destroying foreign microbes. IgM antibodies primarily concentrate antigens for elimination. IgA antibodies are present in bodily secretions, including saliva, tears, sweat and milk; they're critical in controlling overgrowth and colonization of the gastrointestinal tract by microorganisms. IgE and IgD are the final two antibody classes, responsible for initiating allergic reactions and activating naïve B-lymphocytes, respectively.

Class Switching

• The process of class switching allows B lymphocytes to produce the class of antibodies that destroy the invading pathogen most efficiently. For instance, IgG antibodies are best suited for tagging invading bacteria for destruction by phagocytes. This process of coating the pathogen with IgG antibodies to facilitate its rate of destruction, known as opsonization, significantly increases the immune system's ability to control and eliminate a microbe. Class switching occurs by changing the heavy chain molecule of the antibody, a process controlled by an important cell type of the innate immune system: helper T cells. The interface between the innate and adaptive immune system lies in class switching; helper T cells release chemicals that increase the specificity and efficiency of opsonization and destruction by antibodies. As the activated B-lymphocytes proliferate in response to antigen stimulation, the process of class switching allows the progeny to produce the class of antibody most properly suited to elimination by the antigen.