Innate immunity
Innate immunity is otherwise called natural immunity. It is present from birth and it is the inborn capacity of the body to resist the entry of microorganisms into the body. By chance, if the organisms enter the body, innate immunity eliminates them before they cause any disease. This type of immunity represents the first line of defense against any pathogens. Therefore, it is also called nonspecific immunity. Some mechanisms involved in this type of immunity are as follows:
1. Activities of white blood cells and tissue macrophages, which destroy the foreign bodies by means of phagocytosis
2. The enzymes of gastrointestinal tract and the acid is stomach, which destroy the toxic substances or organisms entering digestive tract through foods
3. Lysozyme and some polypeptides, which destroy or inactivate the bacteria.
ACQUIRED IMMUNITY
Acquired immunity is the resistance developed in the body against any specific foreign body like bacteria, viruses, toxins, vaccines or transplanted tissues. So, this type of immunity is also known as specific immunity. This is the most powerful immune mechanism that protects the body from the invading organisms or toxic substances. Lymphocytes are responsible for acquired immunity (Fig. 8).
Main white blood cell function is to participate in defense organism reactions against foreign agents. There exist the natural (non-specific) and specific defense forms.
The non-specific defense is directed to any foreign agent eliminating. The phagocytosis, the complement system and others humoral defense factors are the main types of such reactions. Phagocytosis consists of engulfing the microbes and cells via the formation of the pseudopods followed by endocytosis of the phagocytic vesicle. Next, the endocytotic vesicle is incorporated into the lysosomes of the phagocytes where the microbes and cells are digested by lysosomal enzymes. This phenomenon is adequate to the neutrophiles, monocytes, eosinophiles, macrophages and thrombocytes. In the course of the phagocytosis process we differentiate such stages as the phagocyte approaching to the phagocytized object (or ligand), the ligand contact with the phagocyte membrane, the ligand engulfing, digestion and destruction of the phagocytized object. The phagocytes find their way to the site of injury by chemotaxis or similar guiding mechanisms.
FIGURE 8: Schematic diagram of acquired immunity
Complement system - is a special enzyme system consisting of the proteins (more than 20 types). In includes 9 components (C1…C9). During the activation process some of its components are cleaved in the fragments influencing directly the course of specific and nonspecific defense reactions. There exist the classical and alternative ways of complement system activation. The destruction of foreign and old cells, the phagocytosis and the immune reactions course activates, the vessel wall permeability increases, the blood coagulation hastens at the complement system activation that influence the pathological process.
The other humoral defense factors – defense reactions connected with the action of such substances as lysozyme and interferon. Lysozyme as a protein possesses the enzyme activity suppressing the growth and the development of causative agents and destroying some of the microorganisms. It can be found in nasal mucosa, intestines, salivary secret, lacrimal fluid etc. In small amounts one can find it in the granules of polymorphonuclear leukocytes, in macrophages and when destroyed they fall into the extracellular fluid. Interferon as the globulin of blood plasma can be located in the lymphocytes providing antiviral defense and delaying the cancer cell growth.
Specific defense – immunity – is a reaction complex directed to maintaining the homeostasis on meeting the host’s body with the antigens which are considered as foreign (despite their forming in the organism itself or if they come into it from outside). Under the action of antigen the host body forms the antibodies, activates lymphocytes and thus they get the ability to participate in the immune response. This antigen ability to cause the specific immune response is due to the presence of multiple determinants on its molecule. The active centers of forming antibodies specifically correspond to the determinants like the key to the lock. The antigen interacting with its corresponding antigen forms the immune complex.
The immune organs are divided into central (thymus, bursa of Fabricius, bone marrow) and the peripheral (lymphatic nodes, spleen etc.). There are two categories of acquired immune responces – humoral or antibody-mediated and cell-mediated.
Acquired immunity developed by the entrance of any foreign body or a vaccine is called active immunity.
Types of Acquired Immunity
Two types of acquired immunity:
1) Cellular immunity and
2) Humoral immunity
Cellular Immunity
The cellular immunity is by the activation of T-lymphocytes, which destroy the organisms, entering the body. This is also called cell mediated immunity or T-celled immunity. This is the major defense mechanism against infections by viruses, fungi and few bacteria like tuberculosis bacillus. Cellular immunity is also responsible for delayed allergic reactions and the rejection of tissues transplant from other's body.
Humoral Immunity
Humoral immunity is by the activation of В lymphocytes. This is also called В cell immunity. В lymphocytes act against the invading organisms by secreting antibodies into the blood and lymph. The blood and lymph are body fluids (humors) and the В lymphocytes provide immunity through humors. Therefore, this type of immunity is called humoral immunity. This plays an important role in defense mechanism against the bacterial and viral infections.
ANTIGENS
DEFINITION AND TYPES
The antigens are the substances, which induce specific immune reactions in the body. Antigens are of two types. Those present on the body's own cells are called the auto-antigens or self antigens. The antigens entering the body from outside are known as foreign or non-self antigens.
NON-SELF ANTIGENS
Following are the non-self antigens:
1. The receptors on the cell membrane of microbial organisms such as bacteria, viruses and fungi
2. The toxins from microbial organisms
3. The materials from transplanted organs or incompatible blood cells and
4. Allergens or allergic substances like pollen grains.
The non-self antigens are classified into two types depending upon the response developed against them in the body.
1. The antigens, which induce the development of immunity or production of antibodies (immunogenicity)
2. The antigens, which react with specific antibodies (allergic reactivity).
CHEMICAL NATURE OF THE ANTIGENS
The antigens are mostly the conjugated proteins like lipoproteins, glycoproteins and nucleoproteins.
DEVELOPMENT OF CELL-MEDIATED IMMUNITY