THREE LINES OF IMMUNE DEFENSE IN THE BODY-We often experience that one person may have one cold after another while the other person never catches cold although both have the same chances for exposure. This is because of his resistance towards some infected organisms. In earlier days, immunity was defined as the resistance of the individual to an infected organism. Expanding knowledge about blood groups, tissue grafts and drug reactions has shown that this older concept of immunity is inadequate. In the modern sense, immunity is the ability of the body to recognise, destroy and eliminate antigenic material foreign to its own. Immunity may also be regarded as the result of the fight of two opposite forces—the infected organism on one hand and the host on the other hand. The result may be complete immunity, partial immunity or total absence of immunity. In fact no one ever enjoyed complete immunity or ever had total absence of immunity. Most of us fall in the category of partial immunity. If an individual possess complete immunity it means he is proof against any infection and would not contract any disease. On the other hand if a person possess no immunity he would die of any infection. We do not encounter such situations in life. So most of us are partially immune and the science of immunity is concerned with making the partially immune individual completely immune as far as possible. There are three lines of defense (barriers) in the body that must be overcome by a pathogen before it can establish an infection or disease.

THREE LINES OF IMMUNE DEFENSE IN THE BODY-Here we discuss three lines of defense system of our body.they are protect us from diseases.

Mechanical Barriers (First Line of Defense)

They form a part of innate (natural) immune system. Our body is surrounded by both pathogenic and non-pathogenic organisms which through various portals (water, milk, food, air, vectors etc.) try to enter the body but are stopped by many efficient mechanical barriers, which would restrict the entry of such microbes. These mechanical barriers are:

a) Unbroken skin: Although unbroken skin is a good mechanical barrier and does not permit the organisms to enter the tissues, sometimes the organisms gain their entry through the hair follicle or opening of sweat gland or through the skin if it is torn or there is any cut or wound.


b) The conjunctivae: The conjunctivae are protected by the motion of the eyelids and the constant washing of the tears, still the organisms may enter through the conjunctivae.

c) Mucous membranes and mucous secretions: The mucous secretions of respiratory, digestive and urinogenital organs form a protective covering to mucous membranes. Secretions from the sweat glands, gastric juice and vagina hold many microorganisms and make them lose their infectivity. Peristalsis, that is rhythmic contractions of the intestine, also help in trapping the microbes into mucous and other such secretions. These secretions whether they are acidic (sweat, gastric, juice, vaginal) or alkaline (bile) keep the bacterial flora of the alimentary canal in check. Although the mucous membranes of the genitalia are protected from most bacteria, yet several organisms can invade the body through sexual intercourse (organisms of gonorrhoea, syphilis, AIDS, etc.).

d) Mechanical arrangement of the structures: Turbinate bone in the nose also forms a line of defense. The bacteria when they enter through nose, they either stick to the mucous surface of the nose or they take a lot of time to reach the alveoli because of its structure. By the time they reach the alveoli, most of the organisms die and very few succeed in entering the organ.

e) Other barriers: Coughing, sneezing, shedding tears, perspiring, salivating etc. also help mechanically to remove the organisms. It may be mentioned here that Lysozyme, an enzyme present in most secretions of the body, acts as a chemical barrier.

Phagocytes (Second Line of Defense)

These also form a part of innate immune system. If the microorganisms succeed in passing the first line of defense and enter the deeper tissues, they are attacked by the specific cells of the body called phagocytes or natural killer cells (NK), which include the cells of the reticuloendothelial system (RES). These phagocytes ingest the microbes and destroy them within an hour or two.

Antibodies (Third Line of Defense)

These form a part of adaptive immune system. If the organisms succeed in passing through the second line of defence also, then the body tissues produce special substances known as antibodies which act against the germs and kill them. The antibodies apart from destroying the infective microbes, also neutralize any poisonous substance, which may have been formed.

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