Immunology | Immune system| Immunity

                                    
               Immunity or disease resistance is the ability of an organism to resist the development of a disease.The study of immunity is called immunology,While the infected person with no disease is known as immune.Immune system forms the Third line of defence.The  most peculiar characteristic of immune system is that it can differentiate between 'self'(body's own cells)and 'non-self'(foriegn microbes).

Any foriegn substance which,when enters the body,is capable of stimulating an immune response is called an antigen.The protective chemicals produced by the body in response to antigens are known as antibodies.Antibodies are a class of protiens called immunoglobulins(Igs).So antigens are also called antibody-generating chemicals.The antigens are generally large sized proteinous or polysaccharide molecule present on the walls of bacteria or on the proteinous capsid of viruses.The generally have a molecular weight more than 8,000 daltons.The antibodies are always proteinous in nature and act as neutralins(act as antitoxins to neutralise the toxins)or agglutins(clump the antigenic cells so they get immobilized and engulfed by the phagocytes)or preciptin(combine with and precipitate the antigens to be ingested by the phagocytes)or opsonins(adhere to antigenic cells to be eaten by phagocytes e.g.IgG)

Antigen-Antibody reaction

Antibodies are always antigen specific.Each kind of antigen stimulates the formation of specific antibody.The antigen-antibody reaction is specific and this forms the fundamental characteristic of immune system.This specific reactivity is due to presence of complimentary reactive sites which fit together like the lock and key.But not all the antigens are parts of micro-organisms.Inanimate objects like pollens,white of eggs,drugs,transfused blood cells;transplanted tissues and organs,chemicals etc can also act as antigens.


Leucocytes and macrophages,which form second line of defense,always operate through inflammatory response.According to this response,when the micro-organisms like bacteria,viruses etc.enter the body tissues through some injury,these produce some toxic substances kill tissue cells.The damaged cells release histamine which causes the inflammation characterized by dilation of capillaries and small blood vessels surrounding the injury increasing the blood flow to the injured tissues;infected area becomes red,warm and swollen;and increase in permeability of the capillary wall.Plasma leaks into the tissue spaces so diluting the toxins secreted by the bacteria.
The phagocytes(neutrophils and macrophages)show chemotactic response and are attracted by chemicals released from inflammed area.Some of many tissue products that cause these reactions are  histamine,  bradykinin, serotonin, prostaglandins ,several reaction products of the complement system and blood clotting system,and multiple hormonal substances called lymphokines of sensitized T-cells.The phagocytes move towards infected or injured area,leak into the intersritial spaces and engulf the invading microbes.The dead microbes and blood corpuscles form the pus in the wound region which further increases the response of defensive system of the body.This is called inflammatory response.The intensity of the inflammatory response is usually proportional to the degree of tissue injury e.g.Staphylococci bacteria release extremely lethal toxins so initiating rapid development of inflammatory response and the infection is warded of rapidly but Streptococci bacteria do not cause intense local destruction so inflammatory response is slow and are more harmful.