ABSTRACT
In contrast to nonspecific defense mechanisms, specific immune defense systems at birth are not effective fully and require time to develop after exposure to the infecting agent or its antigens. Specific immunity may be acquired naturally by infection or artificially by immunization.This system also called the adaptive immune system mounts a highly sophisticated and specialized immune response to protect us against specific invaders, and provides long-term protection or immunity from subsequent exposure to those invaders.Adaptive immunity can be divided into two branches: the cellular or cell-mediated immune response, also known as Th1-type response, and the humoral immune response, also known as antibody mediated or Th2-type response. These two interconnected immune functions work in concert through finely tuned checks and balances to mount an appropriate defense. In response to bacterial invasion, B-cells of the humoral arm (Th2) proliferate and produce large amounts of appropriate antibodies that flag invaders for elimination from the body. The cellular (Th1) immune response employs specialized T cells to recognize and destroy host cells showing signs of cancer or infection by viruses or parasites. The relative mobilization of each branch of the immune system depends on the specific disease or condition, and the nature of
the response can be influenced by the pathogen itself and where it enters the body. The balance between the cellular (Th1) and humoral (Th2) arms of the immune system is modulated by a highly integrated network of molecular and cellular interactions driven by cytokines, small proteins that act as intercellular chemical messengers. These cytokines, which are regulated by hormones generated by
the endocrine system, can be classified as either Th1 or Th2 depending on their role. Th1 cytokines such as interleukin 2 (IL-2), interferon gamma (IFN-gamma) and interleukin 12 (IL-12) stimulate the cellular response and suppress the humoral response. Th2 cytokines, such as interleukin 10 (IL-10), interleukin 6 (IL-6) and interleukin 4 (IL-4), stimulate the humoral response and suppress the cellular response.Generally, in healthy individuals the immune system is in homeostasis, or has balanced expression of Th1 and Th2 cytokines. If a foreign invader triggers an adaptive cellular or Th1--type response, the feedback mechanism within the immune system greatly reduces the humoral or Th2-type response. Once the invader is controlled or eliminated, a combination of hormones and cytokines act quickly to return the system back towards homeostasis through the same feedback mechanism [1]. 2.1.2 Immunoglobulins and Immune ResponseAntibodies comprise a family of globular proteins termed immunoglobulins (Ig). Five different classes of immunoglobulins have been identified (IgG, IgM, IgA, IgD, and IgE), based on structural differences in the composition of their heavy chains. Some of the immunoglobulin classes contain subclasses. The most abundant immunoglobulins are IgG, IgM, and IgA. IgE antibodies play a major role in allergic reactions and the role of IgD antibodies is not yet fully understood.
