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Immune system disorders The immune system is a multicomponent interactive system. It effectively protects the host from various infections. But an improper functioning of the immune system can cause discomfort, disease or even death. The improper functions fall into the following major groups : (1) hypersensitivity or allergy, (2) auto-immune diseases, and (3) immunodeficiency. (Immune system disorders) Hypersensitivity Allergies result from an inappropriate and excessive immune response to common antigens. Substances that cause allergies are called allergens ; they include dust, moulds, pollen, certain foods, and some medicines (such as penicillin). Allergy involves mainly IgE antibodies and histamine. Mast cells secrete the histamine. A common manifestation of allergy is asthma. Sometimes an allergen may cause a sudden, violent and fatal reaction in a sensititive individual ; this is called anaphylaxis. (Immune system disorders) Autoimmune diseases Autoimmune diseases result when the...

Genetic basis of organ transplants : Success of organ transplants (ie., Cornea, Kidney, Heart, Liver, Bone marrow) and skin grafts depends on a proper matching of histocompatibility antigens that occur in all cells of the body. Chromosome 6 of mouse contains a cluster of genes known as the major histocompatibility complex (MHC), which in humans is called human leukocyte antigen (HLA) complex. The alleles of HLA genes determine the histocompatibility ie., the compatability between donor and recipient tissues in transplants. (Genetic basis of organ transplants) Process of graft rejection : (Allograft) When the graft or tissue involves two genetically distinct members of the same species, graft rejection occurs because the antigens of the graft and host being different, the immune response of the host rejects the graft. The graft dies, decays and is eliminated from the host. The host also reacts to the graft and shows the following symptoms. 1.Skin rashes, 2. Fluid accumulation in spleen ...

Transplantation immunology The replacement of a diseased organ by a transplant (healthy tissue or organ) is called transplantation. In the early 1940 Medawar explained the nature of graft (transplant) rejection while working with the burn patients of World War II. In his observations the following features were made clear. 1. graft of skin from one region of the body to another in the same patient was easily accepted, 2. grafts obtained from close relatives like brother or sister, were rejected 3. when a second graft was performed, by obtaining the tissue from the same donor, the rejection reaction occurred with greater intensity and speed. The graft or transplant leads to various complications in the host body.  (Transplantation immunology) They are mediated by the host’s immune response. Very often the transplant gets rejected or may lead to graft verses host reaction or disease. Before discussing the nature and implications of this rejection phenomenon let us look into the terms inv...

Antibodies Immunoglobulins Immunoglobulins (Igs) are glycoproteins. Each molecule of Ig consists of two pairs of polypeptide chains of different sizes. The smaller chains are called ‘light’ (L) chains and the larger ones are called ‘heavy’ (H) chains. The L chain has a molecular weight of approximately 25,000 and the H chains is of 50,000. The L chain is attached to the H chain by a disulphide bond. The two H chains are also joined together by S-S bonds, depending on the class of immunoglobulins. The H chains are structurally and antigenically distinct for each class and are designated by the Greek letter corresponding to the immunoglobulin class, as follows : IgG (Gamma) IgA (alpha ) IgM(mu) Igd (delta) and IgE (epsilon) The L chains are similar in all classes of immunoglobulins. They occur in two varieties, kappa (k) and lambda (l). A molecule of immunoglobulin may have either kappa or lambda chains, but never both. Region of polypeptide chains Each heavy and light chains consists of...

Antigenic determinants and epitopes The part of the antibody molecule which makes contact with the antigen is termed the paratope. Consequently, the part of the antigen molecule that makes contact with the paratope is called the epitope. As most antigens are protein in nature, they exist in a folder, three dimensional, tertiary structure. Hence there may be a cluster of amino acid sequences on the three dimensional structure constituting a series of epitopes. Each of these epitope clusters on the antigen are known as an antigenic determinant. Generally the substances, which are capable of eliciting an immune response, that is, the synthesis of antibodies are called antigens. However the above definition of antigens do not infer the cell mediated response. Hence the term immunogen is introduced to denote antigenic substances which elicit both humoral and cell mediated immune response or either humoral or cell mediated response. (Antigenic determinants and epitopes) Antigens may be eithe...

Secondary Lymphoid Organs Mucosa associated lymphoid tissues (MALT) : The mucosa lining the alimentary, respiratory, genitourinary and other lumina and surfaces are constantly exposed to numerous antigens. These areas are endowed with a rich collection of lymphoid cells, either specialized aggregates as the Peyer’s patches or Scattered isolated lymphoid follicles – collectively called the Mucosa associated lymphoid tissues (MALT). Such lymphoid tissues in the gut are called Gut associated lymphoid tissue (GALT) and those in the respiratory tract are called Bronchus associated lymphoid tissue (BALT). MALT contains lymphoid cells as well as phagocytic cells. Both B and T cells are present. The mucosa is endowed with secretory IgA. The mucosal regions afford/confer protection from many enteric and respiratory infections. (Secondary Lymphoid Organs) Antigens The terms immunogen and antigen are often used synonymously. However, these terms imply two closely related entities. The first descr...

Peripheral Lymphoid Organs A. Lymph nodes The lymph nodes are small, round or ovoid bodies placed along the course of lymphatic vessels. They are surrounded by a fibrous capsule from which trabeculae penetrate into the nodes. The node can be differentiated into an outer cortex and an inner medulla. In the cortex are accumulations of lymphocytes (primary follicles) within which germinal centers (secondary follicles) develop during antigenic stimulation. The follicles contain, besides proliferating lymphocytes, dendritic macrophages which capture and process the antigen. In the medulla, the lymphocytes are arranged as elongated branching bands (medullary cords). The cortical follicles and medullary cords contain B lymphocytes and constitute the bursa or bone marrow dependent areas. Between the cortical follicles and medullary cords, there is a broad, intermediate zone called paracortical area which contains T lymphocytes and constitutes the thymus dependent area. Lymph nodes act as a fil...

Structure and Functions of the Immune System The lymphoid system consists of the lymphoid cells (lymphocytes and plasma cells) and lymphoid organs. Based on different roles they perform, lymphoid organs can be classified into central (primary) and peripheral (secondary) lymphoid organs. The central lymphoid organs are lymphoepithelial structures in which the precursor lymphocytes proliferate, develop and acquire immunological capability. In mammals, the bone marrow, the thymus and the bursa of fabricius in birds represent primary lymphoid organs. (Structure and Functions of the Immune System) After acquiring immunocompetence, the lymphocytes migrate along blood and lymph streams, accumulate in the peripheral lymphoid organs and, following antigenic stimulus, effect the appropriate immune response. The spleen, lymph nodes and mucosa – associated lymphoid tissue (MALT) constitute the major peripheral or secondary lymphoid organs. Lymphoidal tissues in the gut ( peyer’s patches), appendix...

Acquired Immunity (Specific immunity) 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. Unique features of the Adaptive immunity (i) Specificity : It is the ability to distinguish differences among various foreign molecules. (ii) Diversity : It can recognize a vast variety of foreign molecules. (iii) 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. (Acquire...

IMMUNOLOGY 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 i...

AIDS – HIV AIDS (HIV) and its control The disease Acquired Immuno Deficiency Syndrome (AIDS) was identified in the year 1981 (December). Early epidemiological studies have established that it is a communicable disease transmitted through sexual contact or through blood and blood products. In 1983 Luc Montagnier at Pasteur Institute, Paris and Gallo at National Institute of Health (NIH) USA isolated the virus that caused AIDS. In 1986, the committee on taxonomy of virus coined the term HIV or Human Immunodeficiency Virus to avoid confusion due to different names being given by different reasearchers. HIV is new member of the Lentivirinae subfamily of human retroviruses. Retroviruses are RNA viruses, which have the capacity to convert their RNA into DNA with the help of an enzyme called reverse transcriptase. Structure of HIV HIV is spherical in shape. Its size is about 100-140 nm. Like any other virus, it is made up of a central icosahedral capsid core containing the genetic materi...

Antibiotics and Chemotherapy Antibiotics are special and unique type of chemotherapeutics agents obtained from living organisms such as bacteria or fungi. The word antibiotic refers to a metabolic product of one microorganism that in very small amounts is detrimental or inhibitory to other microorganisms. Antibiotics are special and unique type of chemotherapeutics agents obtained from living organisms such as bacteria or fungi. The word antibiotic refers to a metabolic product of one microorganism that in very small amounts is detrimental or inhibitory to other microorganisms. The first antibiotic that was discovered was Penicillin from the fungus, Pencillium sp. by Alexander Flemming in 1929. Since then hundreds of antibiotic substances have been isolated. Antibiotics are of two types. 1. Broad spectrum antibiotics can destroy or inhibit many different species of pathogens. 2. Narrow spectrum antibiotics can destroy specifically some or few species of pathogens. The mode of action of...

Antimicrobial Resistance Pathogenic microbes are provided with their own mechanisms to establish infection in the host animals and humans. Similarly the hosts body has a number of defense mechanisms to mount resistance against the invasion and to prevent infection of pathogens. The antimicrobial response of hosts may be natural, non-specific and specific. Natural resistance includes species resistance, racial resistance and individual resistance. (Antimicrobial Resistance) Chemotherapy The control and treatment of infectious diseases with a chemical compound or drug is called chemotherapy. The chemical compounds and drugs are called chemotherapeutic agents. A good chemotherapeutic agent posses the following characteristics: 1. It destroys or prevents the activity of a disease causing pathogen, without injuring the host tissues 2. It is able to penetrate the cells and tissues of the host and can encounter the pathogens in effective but safe concentrations or dosage. 3. It leaves the hos...

Pathogenecity of Microorganisms Pathogenecity refers to the ability of microorganism to cause the disease in animals and humans. Infectious diseases more often result due to the interactions between the disease producing pathogenic microorganisms and host organisms. The Pathogenic adaptations The Pathogenecity of the microbes is due to several phenomena or adaptations. 1. Pathogens are able to selectively attach to the external surfaces such as the skin and conjunctiva or the internal surfaces such as the mucus membranes of the respiratory, gastrointestinal or urinogenital tracts. 2. They also penetrate the above body surfaces and gain access to the internal tissues. 3. In some infections, the pathogen may remain localized, growing near its point of entry into the body. 4. Some pathogens become widely distributed in different tissues or organs. This is called generalized infections. 5. Some other pathogens can grow within the cells of host, causing severe disturbances to normal physiol...

Protozoan microbiology Protozoans are eukaryotic, single celled organisms, which are predominantly microscopic in size. The majority are between 5 to 250 microns in diameter. Protozoan microbiology is mostly concerned with the disease aspects in humans. Some well known protozoan diseases in human beings are the intestinal amoebiasis, African sleeping sickness, and malaria. Several flagellate protozoans are responsible for diseases in both children and adults. Giardia intestinalis is associated with diarrhoea in children. Trichomonads are found in the mouth and cause gingivitis. A. Plasmodium and Malaria (Protozoan microbiology) Malaria is a mosquito borne disease caused by the protozoan, Plasmodium. The life cycle of malarial parasite comprises two phases namely the asexual phase in man and the sexual phase in the female Anopheles mosquito (Definitive host). Four different types of Malaria are recognized on the basis of period of recurrence of fever. They are 1. Tertian, Benign Tertian...

Bacterial Diseases A. Salmonella and Human Diseases: (bacterial diseases) Salmonella are pathogenic bacilli which cause three kinds of infection to humans viz., enteric fever (Typhoid or Paratyphoid), Gastroenteritis and Septicemia. Typhoid fever is caused by S.typhi. It is transmitted via Pathogen contained food and water. The disease is characterized by a continued fever, inflammation of the intestine, formation of intestinal ulcers and enlargement of the spleen. Gastroenteritis is caused by Salmonella, but most commonly by S.choleraesuis. The bacteria reach the blood stream from the intestinal tract, where it multiplies. It causes recurring high fever, chills, loss of appetite and weight loss. When the bacteria infect the organs from blood they can cause meningitis, pneumonia, abscesses, nephritis, osteomyelitis, or endocarditis, etc. B. Cholera : (bacterial diseases) Cholera is caused by Vibrio cholere. It is a disease of antiquity and has been the cause of untold sufferings and de...

Bacterial Genetics In bacteria the cells have a single circular strand of DNA. It is not associated with proteins as are eukaryotic chromosomes. The bacterial genes, like the eukaryotic genes possess the features of replication, phenotype expression, mutation and genetic recombination etc. In bacteria the genetic recombination results from three types of gene transfer viz., conjugation, transduction and transformation. (Bacterial Genetics) Conjugation involves the transfer of some DNA from one bacterial cell to another followed by the separation of the mating pair of cells. In this, large segments of the chromosomes and in special cases the entire chromosome may be transferred. Bacterial transformation is a process in which cell free or naked DNA containing the genetic information is transferred from one bacterial cell to another. It was discovered by an English health officer, Griffith in 1928. The transforming principle was identified as DNA by Avery Macleod and Mc Carthy in 1944. (B...