Animals and human beings are continually exposed to various infectious agents like bacteria, viruses, fungi and parasites. It has long been noticed that survivors of certain diseases, e.g., measles, are not attacked by the same disease again. Clearly, these people have become immune to the concerned disease. The system of animal body, which protects it from various infectious agents and cancer, is called Immune system. A study of the immune system is known as Immunology. This chapter introduces the fundamental concepts of immune system and their use for the improvement of human health and welfare.
The Latin term ' Immunis', meaning ' exempt' or ' freedom', gave rise to the English word immunity. It refers to all the mechanisms used by the body for protection from environmental agents that are foreign to the body. These agents may be microorganisms or their products, certain food items, chemicals, drugs and pollen grains. Immunity is of two types : (a) innate, and (b) acquired immunity.
the site of infection. These monocytes get converted into macrophages. These cells are provided with bacteriolytic enzymes and free radicals, which destroy the pathogens.
Besides the phagocytes, natural killer cells (NK cells) (T Lymphocytes) kill virus-infected cells and some tumour cells of the body by creating perforin-lined pores in the plasma membrane of the target cells. These pores allow entry of water into the target cell, which then swells and bursts.
Acquired immunity, also known as adaptive or specific immunity, is capable of recognizing and selectively eliminating specific microorganisms. Acquired immunity is found only in verterbrates. It supplements the protection provided by innate/natural immunity. It is generated in response to an exposure or encounter to the microorganisms in question. Specific defence mechanisms require several days to be activated, following the failure of non-specific defence mechanisms.
(i) Specificity : It is the ability to distinguish differences among various foreign molecules.
(ii)Diversity : It can recognize a vast variety of foreign molecules.
Discrimination between Self and Non-self : It is able to recognize and respond to molecules that are foreign (non-self) to the body. At the same time, it can avoid response to those molecules that are present within the body (self antigens) of the given animal.
(iv) Memory : When the immune system encounters a specific foreign agent, e.g., microbe, for the first time, it generates an immune response and eliminates the invader. The immune system retains the memory of this encounter for a prolonged interval. As a result, a second encounter with the same microbe evokes a heightened immune response.
Specific immunity employs two major groups of cells :
(a) lymphocytes, and (b) antigen presenting cells. A healthy individual possesses about a trillion of lymphocytes. The lymphocytes are of two types viz., T- lymphocytes or T-cells and B - lymphocytes or B - cells. Both the types of lymphocytes, as well as the other cells of the immune response, are produced in bone marrow. The process of their production is called haematopoiesis. Some immature lymphocytes, destined to become thymocytes, migrate via blood to the thymus, where they mature and differentiate as T - cells. The B- cells, on the other hand, mature in the bone marrow itself. The B and T cells, together, generate two types of specific immunity, viz., (a) cell-mediated and (b) antibody-mediated or humoral immunity respectively.
Cell-mediated immunity is the responsibility of a subgroup of T cells, called cytotoxic T lymphocytes (CTLs). An activated cytotoxic lymphocyte is specific to a target cell, which has been infected, and kill the target cell by a variety of mechanisms. This prevents the completion of life cycle of the pathogen and its growth, since it depends on an intact host cell to do that. Cell-mediated immunity is also involved in killing of cancer cells.
Antibody mediated or humoural immunity involves the synthesis of specific antibody molecules called immunoglobulins by the B-lymphtocytes. Each antigen has many different antigenic determinants, each of which matches a specific antibody and binds to it. The B cells, direct the antibody-mediated immunity. The antibody molecules (Igs) may be bound to a cell membrane in the form of receptors or they may remain free. The free antibodies have three main functions viz., 1. agglutination of particulate matter, including bacteria and viruses, 2. opsonisation or coating over bacteria to facilitate recognition and phagocytosis by the phagocytes and 3. neutralization of toxins released by bacteria.
Adaptive immunity may be active or passive. Active immunity is due to the immune response generated in the individual in question by a pathogen or vaccine, whereas passive immunity is conferred by transfer of immune products, like antibodies, etc., from an individual into a non-immune individual.
Every antigen is processed by antigen presenting cells(APC), like macrophages, B lymphocytes and dentric cells. The processed antigen is presented on the surface of these cells. A subgroup of T cells called T helper cells, specifically interacts with the presented antigen and becomes activated. The activated T helper cells then activate B cells, and a subgroup of T cells called cytotoxic lymphocytes (CTLs), in a specific manner. The activated B and cytotoxic lymphocytes proliferate to produce clones. All the cells of a clone can recognize the same antigen and eliminate it.