Veterinary IMMUNOLOGY (Animal)

IMMUNOLOGY

(Animal)

All living organisms are continuously exposed to substances that are capable of causing them harm. Most organisms protect themselves against such substances in more than one way with physical barriers, for example, or with chemicals that repel or kill invaders. Animals with backbones, called vertebrates, have these types of general protective mechanisms, but they also have a more advanced protective system called the immune system. The immune system is a complex network of organs containing cells that recognize foreign substances in the body and destroy them. It protects vertebrates against pathogens, or infectious agents, such as viruses, bacteria, fungi, and other parasites.

 Lymphocytes - Heart of the Immune System

Lymphocytes  - a class of white blood cells- are theprincipal active components of the adaptive immune system.

The other components are antigen-presenting cells, which trap antigens and bring them to the attention of lymphocytes so that thev can mount their attack.

When an antigen invades the body, normally only those lymphocytes with receptors that fit the contours of that particular antigen take part in the immune response. When they do, so-called daughter cells are generated that have receptors identical to those found on the original lymphocytes. The result is a family of lymphocytes, called a lymphocyte clone. with identical antigen-specific receptors. lymphocytes are made from stem cells in the bone marrow lymphocytes then undergo a second stage of development, or processing, in which they acquire their antigen-specific receptors.

 By chance, some lymphocytes are created with receptors that happen to be specific to normal, healthy components of the body. Fortunately, a healthy immune system purges itself of these lymphocytes, leaving only lymphocytes that ignore normal body components but react to foreign intruders. If this purging process is not completely successful, the result is an autoimmune disease in which the immune system attacks normal components of the body as though they were foreign antigens, destroying healthy molecules, cells, or tissues. Antibodies attack antigens by binding to them. Some antibodies attach themselves to invading microorganisms and render them immobile or prevent them from penetrating body cells. In other cases, the antibodies act together with a group of blood proteins, collectively called the complement system, that consists of at least 30 different components. In such cases, antibodies coat the antigen and make it subject to a chemical chain reaction with the complement proteins. The complement reaction either can cause the invader to burst or can attract scavenger cells that eat the invader.

Cytotoxic T cells destroy cells infected with viruses and other pathogens and may also destroy cancerous cells. Cytotoxic T cells are also called suppressor lymphocytes because they regulate immune responses by suppressing the function of helper cells so that the immune system is active only when necessary.

The receptors of T cells are different from those of B cells because they are trained to recognize fragments of antigens that have been combined with a set  of molecules found on the surfaces of all the body's cells. These molecules are called MHC molecules. As T cells circulate through the body, they scan the surfaces of body cells for the presence of foreign antigens that have been picked up by the MHC molecules. This function is sometimes called immune surveillance.

  

Immune Response

When an antigen enters the body, it may be partly neutralized by components of the innate immune system. It may be attacked by phagocytes or by preformed antibodies that act together with the complement system. Often, however, the lymphocytes of the adaptive immune system are brought into play. If lymphocytes encounter an antigen trapped by the antigen-presenting cells of the lymphoid organs, lymphocytes with receptors specific to that antigen stop their migration and settle to mount an immune response locally. As these

lymphocytes accumulate in the affected lymphoid tissue, the tissue often becomes enlarged for example, the lymph nodes in the groin become enlarged if there is an infection in the thigh area.

Antigen-presenting cells degrade antigens and ofteneliminate them without the help of lymphocytes. If there are too many antigens for them to handle alone, however, the antigen-presenting cells secrete IL- 1 and display fragments of the antigens to alert thehelper T cells. The IL-1 facilitates the responsiveness of T and B cells to antigens and, if released in large amounts, can also cause fever and drowsiness. Helper T cells that encounter IL-  1 and fragments of antigens transform into cells  called lymphoblasts, which then secrete a variety of interleukins that are essential to the success of the immune response. TheIL-2 produced by helper T cells promotes the growth of cytotoxic T cells, which may be necessary to destroy tumorous cells or cells infected with viruses. The IL-3 increases the production of blood cells in the bone marrow and thus helps to maintain  an adequate supply of the lymphocytes and lymphocyte products necessary to fight infections. Helper T cells also secrete interleukins that act on B cells, stimulating them to divide and to transform intoantibody-secreting plasma cells. The antibodies thenperform their part of the immune function.

The process of inducing an immune response is called immunization. It may be either natural through infection by a pathogen or artificial through the use of serums or vaccines. The heightened resistance acquired when the body responds to infection is called active immunity. Passive immunity results when the antibodies from an actively immunized individual are transferred to a second, nonimmune subject. Active immunization, whether natural or artificial, is longer-lasting than is passive immunization because it takes advantage of immunologic memory