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Defense in the body is provided by the immune system into distinct ways: the adaptive system and the innate system. The innate system tries to prevent the body from all invaders. The adaptive system attracts against specific intruders. In the innate system, the first line of defense is given by the mucosae and skin, which behave as physical barriers to invasion.

First Line Defense

The keratinized, thick stratum corneum provides impenetrable barriers to microorganisms that try to enter the body through intact skin. Mucous membranes protect the body from potential invaders. It produces mucus to trap microorganisms that try to enter the body. Cilia lining internal passages sweep mucus into the digestive tract or out of the body. Mucus also has lysozyme.

The alimentary canal is a mechanism of defense which fights harmful bacteria in the stomach by producing gastric acids. Moreover, the plentiful population of microorganisms found in the lower colon releases essential vitamins, indirectly providing other defense mechanisms.

The immune system is a defense mechanism which is a complex phenomenon that comprisies antibodies and antigens. Antibodies are materials manufactured by the body in response to an antigen. The antigen is referred to as foreign protein that releases immune reaction in the body.

The eye mucus membrane is another defense mechanism. It protects from infections by lacrimal secretion such as tearing that contain wandering lysozyme and phagocytes. These are enzymes that destroy bacteria.

Second Line Defense

The second line of defense comprises antimicrobial proteins and phagocytic cells, which try to counter the spread of any intruders that happen to pass through the first line of defense. Phagocytic cells go through the body tissues looking for intruders that have broken the surface barriers. The main phagocytes are macrophages, which are monocytes that have entered the tissues from bloodstream.

Heat chemical, infections and physical trauma provide the inflammatory response in tissues. Inflammation comprises swelling, redness, heat and pain in the infected or injured areas.

Several of antimicrobial proteins help in defense by hindering their ability to destroy pathogens directly. Fever, which can be defined as an abnormally soaring body temperature, is an efficient response to infections. Exposure to foreign invaders leukocytes releases chemical known as pyrogens. This chemical causes the body temperature to rise.

These are lymphocytes that mainly detect and kill cancer cells and cells infected by viruses. Other lymphocytes act against certain targets. NK cells are less particular and kill a broad range of target.

B and T cells

B lymphocytes and T lymphocytes play significant roles in the adaptive immune system. Immature lymphocytes are primary identical, but at some instances, they become immunocompetent, thus able to identify specific antigens. Immunocompetent in the thymus is as a result of T cells form from lymphocytes. Immunocompetent in the bone marrow comes from B cells form from lymphocytes.

Humoral immune response is initiated when B cells contact with antigen. The B cells that bid to the antigen are activated then and they start to multiply and divide. The word that is used to refer to proliferation of the activated B cell is known as clonal selection. T cell is activated by antigen and start to multiply to form a host of clones. The basic method of clonal selection is used both to describe B cells and T cells.

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