Concepts of Immunology :
Immunology is the science which deals with the study of immune system, immune responses to foreign substances and their role in resisting infection. The study of structure and function of immune system is called basic immunology. Other branches are Clinical immunology (vaccination/immunisation, organ transplantation/organ grafting, blood banking, immunopathology), Laboratory immunology (testing of cellular and humoral immune function), Serology etc. The human immune system consists of lymphoid organs, tissues, widely distributed cells and molecules like antibodies. The most peculiar characteristics of immune system are i) It can differentiate between self (own body cells or molecules) and non-self (foreign molecules), ii) It possesses immunological memory.
Any foreign substance invading body and capable of stimulating an immune response, is called an antigen. The protective chemicals produced by immune cells in response to antigens are called antibodies. Immune system is a complex system including cellular and molecular components which provide defence against infections agents.
The term immunity (L. immunis - exempt or freedom) refers to the general ability of a body to recognize, neutralize/destroy and eliminate foreign substances or resist a particular infection or disease.
Types of Immunity :
Immunity is classified into two basic types : I) Innate immunity II) Acquired immunity
I) Innate immunity : Inner immunity is also called natural immunity. It is present from birth. Innate immunity is the inborn capacity of the body to resist the pathogen. It does not depend on previous exposure to foreign substances. It is also called non-specific immunity, because it comprises of all those defence elements.
it consists of the following types of barriers : 1) Anatomical, 2) Physiological, 3) Phagocytic and 4) Inflammatory barriers.
1) Anatomical barriers (Physical barriers) :
These barriers prevent entry of the microorganisms into the body. They include skin and mucous membrane. Mucous membrane secretes mucus and the mucus coating of the epithelial lining of the respiratory, gastrointestinal and urinogenital tract help in trapping microbes entering the body.
2) Physiological barriers :
Physiological barriers like body temperature, pH and body secretions prevent growth of many invading pathogenic micro-organisms. Elevated body temperature i.e. fever inhibits the growth of many pathogens. Acidity of gastric juice in stomach kills most of the ingested micro-organisms. Lysozyme is a bacteriolytic enzyme, present in tears, which digests bacterial cell walls. Certain kinds of cells, when infected with virus, release interferons (glycoproteins). Interferons make the surrounding cells resistant to viral infection.
3) Phagocytic barriers (cellular barriers) :
These bring about phagocytosis of invading microbes. Phagocytosis is an important mechanism of innate immunity. Neutrophils and monocytes are phagocytic leucocytes which engulf and destroy invading microbes. Macrophages are large irregular shaped cells that engulf microbes and cellular debris. e.g. Kupffer's cells of liver.
4) Inflammatory barriers :
An infection or tissue injury often causes redness, swelling, pain and the production of heat that may result in fever. Such localized manifestation is called inflammatory response. This response is due to release of certain chemicals such as histamines and prostaglandins, by damaged mast cells of connective tissue and basophils of blood. These chemicals dilate and make the blood capillaries more permeable in the region of tissue injury. The vascular fluid comes out of the blood vessels. This fluid contains serum proteins which kill bacteria.
II) Acquired Immunity : Ability of the body to defend itself invading foreign agents like bacteria, viruses, toxins and transplanted tissues is called specific resistance. This resistance or immunity that an individual acquires during life is called acquired immunity. (adaptive immunity or specific immunity). Acquired immunity is found only in vertebrates. It supplements the protection provided by innate immunity. It requires several days to become activated.
Unique features of acquired immunity :
i) Specificity : It is the ability to differentiate various foreign molecules. It is specific for each type pathogen.
ii) Diversity : It can recognize a vast variety of diverse pathogens of foreign molecules.
iii) Discrimination between self and non-self : It is able to differentiate between own body cells (self) and foreign cells or molecules (non-self).
iv) Memory : When the immune system encounters a specific pathogen for the first time, it generates immune response and eliminates the invader. The immune system retains the memory of this encounter. As a result, a second encounter with same pathogen brings about quicker and stronger immune response.
Types of acquired immunity :
Acquired immunity may be active or passive.
1. Acquired active immunity :
It is the resistance developed by an individual as a result of an antigenic stimulus of invading pathogens or vaccine. This immunity is acquired by activating immune system of the body and results in production of antibodies. It is of two types:
i) Natural acquired active immunity : Immunity acquired due to infection is called natural acquired active immunity. It is developed after entry of pathogens in the body. For e.g., person who has recovered from an attack of measles develops natural acquired active immunity to measles for the life time.
ii) Artificial acquired active immunity : This immunity is acquired artificially by vaccination. Vaccines contain dead or live but attenuated (artificially weakened) pathogens or toxoids consisting of microbial components or toxins secreted by the pathogens. Vaccine is introduced into the body to stimulated the formation of antibodies by the immune system. e.g., polio vaccine, BCG vaccine, etc.
2. Acquired passive immunity :
Passive immunity is acquired when ready-male antibodies are received by the body. Passive immunity can be acquired either naturally or artifically.
i) Natural acquired active immunity : Before birth maternal antibodies are transferred from mother to foetus through placenta. After birth antibodies are transferred from mother to infant through colostrum. (IgA) The antibodies received by child from mother remain in the body for a short time. Therefore, natural acquired passive immunity is short lived.
ii) Artificial acquired active immunity : This immunity is developed by injecting previously prepared antibodies using serum from humans of animals. For e.g., antibodies obtained from hyperimmunised horses are injected to humans against rabies pathogens.
Cells of acquired or specific immune system:
Two major groups of cells are involved in acquired immunity which are a) lymphocytes b) antigen presenting cells.
a) Lymphocytes :
These are the chief cells of specific immune system of the body. A healthy man possesses a trillion lymphocytes. They are of two types : T-lymphocytes (T-cells) and B-lymphocytes (B-cells). Both the types of lymphocytes are produced from stem cells in bone marrow by a process called haematopoiesis. While differentiating from stem cells, some immature lymphocytes, destined to become thymocytes, migrate via the blood to the thymus, where they mature as T-lymphocytes. On the other hand B-lymphocytes divide and mature lymphocytes circulate in the body fluids and many of them reside in lymphoid organs such as the lymph nodes, spleen etc. On being sensitized both types of lymphocytes undergo division. A sensitized T. lymphocyte produces a clone of T-cells having different functions. B-lymphocytes produce specific plasma cells which produce antibodies. T-lymphocytes are responsible for cell-mediated or cellular immunity while B-lymphocytes generate antibody-mediated or humoral immunity.
Mechanism of action of T-lymphocytes to antigens: On coming in contact with an antigen a T-lymphocyte forms a clone of T-cells which are similar but they perform different functions. The clone has four types of T-lymphocytes: i) T-Helper T-cells ii) Killer T-cells iii) Suppressor T-cells iv) Memory T-cells.
i) Helper T-cells : Sensitized helper T-cells produce lymphokines for performing several types of functions like proliferation of other T-cells, stimulation of B-lymphocytes, attraction of macrophages etc.
ii) killer T-cells or Cytotoxic T-cells : They directly attack and destroy invading microbes, infected body cells and cancer cells. Killer T-cells bind to infected cell and secrete perforins. Then perforins produce a hole in infected cell. It also releases cell killing substances, hence the name cytotoxic T-cell.
iii) Suppressor T-cells : These cells suppress entire immune system to attack on the own body cells.
iv) Memory T-cells : These cells were previously sensitized and retain the sensitization for future.
a) Mechanism of action of B-lymphocytes to antigens:
B-lymphocytes are sensitized both directly by antigens as well as by helper T-cells. Activated B-lymphocytes multiplies very fast to produce to produce clone of plasma cells and memory B-cells. The plasma cells produce specialized glycoproteins called antibodies which are passed through body fluids (humor) like blood and lymph. The antibody molecules may bind to a cell membrane or they may remain free. The free antibodies have three main functions: Agglutination of particulate matter, including bacteria and viruses. The immobilized mass is then engulfed by phagocytes. Opsonisation or coating of bacteria to facilitate their subsequent phagocytosis by macrophages and neutralization of toxins released by bacteria e.g., tetanus toxin. Each antibody is specific for a particular antigen.
b) Antigen Presenting cells:
These are specialized cells which include macrophages (monocytes as blood macrophages and histocytes as tissue macrophages), B-lymphocytes and dendritic cells (e.g., Langerhans cells of epidermis of skin). Antigen presenting cells engulf invading pathogens and process the antigens. Then the processed antigens are presented on their surface. These cells are able to deliver a co-stimulatory signal that is necessary for helper T-cell activation.