Immunity and Types of Immune Response
Immunology is a science investigating immunity.
- Immunity is a great set of reactions between the immune system and particular specific active substances (antigens).
- The immune response plays an essential role in body protection against a tremendous variety of foreign agents.
- Nevertheless, it should be noted that the broad scope of primary defensive reactions is not directly mediated by the host immune system.
- These reactions are controlled by the general mechanisms of species hereditary resistance as well as by the common pathways of non-specific resistance.
Types of Immune Response
- It presumes the insusceptibility of members of certain species to the diseases that affect another species.
- The versatile mechanisms of hereditary resistance include the lack of specific receptors on the target cells required for adhesion and propagation of the causative agents, and conversely, the organization of the host metabolic pathways intolerable for certain microbial pathogens or the production of substances that naturally block the reproduction of pathogenic agents.
- As an example of the species-inherited resistance is the insusceptibility of humans to many zoonotic diseases such as cattle plague or chicken cholera. Similarly, animals are not affected by many human infections like enteric fever, scarlet fever, syphilis, measles, etc.
- The acquisition of hereditary resistance is the result of a long evolution of complex relationships between the host and pathogenic microorganisms.
- It hinges on the biological peculiarities of a certain species, which were formed in the course of evolution and natural selection, species variation and continual adaptation to the environmental conditions.
Non-specific host resistance
- Likewise, there is a vast number of reactions of non-specific host resistance that actively participate in body protection.
- They comprise the barrier functions of the skin and mucosal tissues, body temperature reactions (fever), mucociliary clearance of pathogens by epithelial cells and mucosal secretions, microbicidal activities of tears, saliva, secretions of sweat and sebaceous glands, acid contents of gastric juice, the elimination of pathogens with body excretions, and some other similar activities.
- By contrast to non-specific resistance, the immune response is triggered by specific foreign substances and agents.
According to the mechanism of the development, the immunity is divided into two main types: innate and acquired (or adaptive).
Both these types include cellular (maintained by specialized immune cells) and humoral immune reactions (the latter are promoted by molecular soluble immune factors present in biological fluids of the host).
- It encompasses immune cells and molecules, which have arisen regardless of prior antigenic challenge.
- Having been created as the first line of the immune defence, they are responsive against the broad groups of foreign agents and substances.
- Hence, the reactions of the innate immunity are largely non-specific or demonstrate low or limited specificity towards the most common structures that are shared by a vast number of pathogenic agents. This brings the innate immunity closer to the non-specific host resistance.
Acquired (adaptive) immunity
- By contrast, cellular and humoral reactions of acquired (adaptive) immunity need primary antigenic stimulation triggered by the exposure of the immune system to the specific antigen (Ag).
- Thus, acquired immunity demonstrates high specificity and selectivity in response to invaded foreign agents or structures.
- Acquired immunity is commonly divided into natural and artificial immunity.
Both can be further sub-divided into active and passive immunity.
- Acquired active immunity is maintained by active production of special defensive factors (various molecules and cells) by the body itself after antigenic exposure.
- Acquired passive immunity depends on the external source of defensive factors delivered into the host by different ways.
Natural active immunity
- It is acquired following manifested or latent infectious disease or infectious process.
Natural passive immunity
- It is observed mainly in newborns and infants; it is acquired from the mother through the placenta in the period of fetal development or maintained with mother’s milk in breastfeeding. The duration of passive immunity of the newborn is short. After about six months this immune state disappears and children become susceptible to many infections (measles, diphtheria, scarlet fever, etc.).
Artificial active immunity
- It is triggered by active immunization (vaccination with vaccines or toxoids).
- In turn, artificial passive immunity is reproduced by passive immunization (administration of immune sera or antibodies).
Each of immune reactions includes local and generalized immune response.
Local response develops in different body compartments as the function of local specialized immune tissues or resident cells.
- In the generalized immune response, all of the immune subsystems are activated.
- According to the origin of a foreign agent (antigen) that elicits specific immune reactions, the immune response is divided into anti-infectious and non-infectious immunity.
Anti-infectious immunity is stimulated by various microbial antigens. There are several kinds of anti-infectious immunity.
Anti-bacterial immunity caused by bacteria may be sterile or non-sterile. In case of sterile immunity, the host immune response remains stable despite antigen complete elimination.
Non-sterile immunity is maintained by residual microbial cells retained in the body after the infection.
Anti-toxic immunity is induced against the toxins of bacteria, anti-viral – against viruses; anti-fungal is caused by different fungi, anti-parasitic – by various parasites.
Non-infectious immunity is directed against non-infectious antigens. Due to the nature of the antigen, it is divided as follows:
– autoimmunity comprises immune reactions provoked by self-antigens of the body; it arises because of the self-tolerance breakdown;
– transplantation immunity develops after allo- or xenotransplantations; it is mediated predominantly by the antigens of major histocompatibility complex (human leukocytes antigens or HLA in humans and H-2 antigens in mice);
– anti-tumour immunity arises against tumor antigens expressed on cytoplasmic membranes of cancer cells; these antigens emerge due to the genetic instability of tumor genome;
– reproductive immunity encompasses the reactions in “fetus-mother” system in the period of pregnancy and embryogenesis; in particular, they may develop against the fetal antigens, encoded by genes inherited from father.