difference between the immunity obtained from breast milk and the immunity obtained through immunisation or vaccination
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The environment contains a wide variety of potentially harmful organisms (pathogens), such as bacteria, viruses, fungi, protozoa and multicellular parasites, which will cause disease if they enter the body and are allowed to multiply. The body protects itself through a various defence mechanisms to physically prevent pathogens from entering the body or to kill them if they do.
The immune system is an extremely important defence mechanism that can identify an invading organism and destroy it. Immunisation prevents disease by enabling the body to more rapidly respond to attack and enhancing the immune response to a particular organism.
Each pathogen has unique distinguishing components, known as antigens, which enable the immune system to differentiate between ‘self’ (the body) and ‘non-self’ (the foreign material). The first time the immune system sees a new antigen, it needs to prepare to destroy it. During this time, the pathogen can multiply and cause disease. However, if the same antigen is seen again, the immune system is poised to confine and destroy the organism rapidly. This is known as adaptive immunity.
Vaccines utilise this adaptive immunity and memory to expose the body to the antigen without causing disease, so that when then live pathogen infects the body, the response is rapid and the pathogen is prevented from causing disease. Depending on the type of infectious organism, the response required to remove it varies. For example, viruses hide within the body’s own cells in different tissues, such as the throat, the liver and the nervous system, and bacteria can multiply rapidly within infected tissues.
Lines of defence
The body prevents infection through a number of non-specific and specific mechanisms working on their own or together. The body’s first lines of defence are external barriers that prevent germs from entering. The largest of all is the skin which acts as a strong, waterproof, physical barrier and very few organisms are able to penetrate undamaged skin. There are other physical barriers and a variety of chemical defences. Examples of these non-specific defences are given below:
Skin - a strong physical barrier, like a waterproof wall
Mucus – a sticky trap secreted by all the surfaces inside the body that are directly linked to the outside, also contains antibodies and enzymes
Cilia – microscopic hairs in the airways that move to pass debris and mucus up away from the lungs
Lysozyme – a chemical (enzyme) present in tears and mucus that damages bacteria
Phagocytes – various cells that scavenge and engulf debris and invading organisms, which form part of the surveillance system to alert the immune system of attack
Commensal bacteria - bacteria on skin and gut that compete with potentially harmful bacteria for space and nutrients
Acid - in stomach and urine, make it hard for any germs to survive
Fever – elevated body temperature making conditions unfavourable for pathogens to survive
The immune response
An immune response is triggered when the immune system is alerted that something foreign has entered the body. Triggers include the release of chemicals by damaged cells and inflammation, and changes in blood supply to an area of damage which attract white blood cells.
White blood cells destroy the infection or convey chemical messages to other parts of the immune system. As blood and tissue fluids circulate around the body, various components of the immune system are continually surveying for potential sources of attack or abnormal cells.
Antigens and antibodies
Antigens are usually either proteins or polysaccharides (long chains of sugar molecules that make up the cell wall of certain bacteria). An antigen is a molecule that stimulates an immune response and to which antibodies bind – in fact, the name is derived from “antibody generators.” Any given organism contains several different antigens. Viruses can contain as few as three antigens to more than 100 as for herpes and pox viruses; whereas protozoa, fungi and bacteria are larger, more complex organisms and contain hundreds to thousands of antigens.
An immune response initially involves the production of antibodies that can bind to a particular antigen and the activation of antigen-specific white blood cells.
Antibodies (immunoglobulins; Ig) are protein molecules that bind specifically to a particular part of an antigen, so called antigenic site or epitope. They are found in the blood and tissue fluids, including mucus secretions, saliva and breast milk. There are five classes of antibody – IgG, IgA, IgM, IgD and IgE, which have a range of functions. They can act as ‘flags’ to direct the immune system to foreign material for destruction and form part of the innate / humoral immune response. Normally, low levels of antibodies circulate in the body tissue fluids. However, when an immune response is activated greater quantities are produced to specifically target the foreign material.
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