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       It is similar for most of infections.It checks the entry of pathogens with the help of certain physical barriers or destroys the microbes through inflammatory response.It includes two types of defence:

(A) First line of defence or non-specific external defence:It is formed of mechanical barriers(the sheating that prevent the acces of microbes to inside the body,sweat,sebum etc.); lysozomes containing secretions and certain chemical compounds of blood which destroy the foreign organisms or their toxins.
            (a) Skin : It forms the non-specific external defence. The outer Layer of epidermis is a horny layer , called Stratum corneum , and its formed of dead an flat cells. It is germs-proof layer and acts as physical barrier to entry of bacteria and viruses .
           Sebum(oil) and sweat,secreted by sebaceous(oil)and sudoriferous(sweat)glands of skin respectively,make the skin acidic(3 to 5 pH)due to presence of lactic acid and fatty acids.Some useful bacteria of skinalso release acids and other metabolic wastes which check the growth of pathogens on the surface of skin.The sweat and sebaceous secretion of the skin contain bactericidal and fungicidal substances which kill the bacteria and fungi respectively. This shows that skin is a self-disinfecting organ.
Recently,it has beenreported that skin has tissue macrophages which divide in situ
 and form more macrophages when an infection and local inflammation occurs in the subcutaneous tissues.Thee macrophages attack and destroy the microbes.
            (b)Lysozyme containing secretions.The mucous membrane lining the nasal chamber,throat and trachea secretes mucus which contains lysozyme(a lysosomal mucolytic polysaccharide and is bactericidal)which kills the bacteria by causingdissolution of their lipid membrane.The mucosa of trachea is also ciliated to prevent the entry of dust particles or any foreign agent.Many bacteria are trapped in this mesh of hair and are expelled out with mucus into the pharynx by outward beating of cilia.From pharynx,these bacteria are swallowed and are killed by acidic gastric juices or expectorated from pharynx.
            There are a number of chemical substances which kill the bacteria which tend to enter our body:
(i)Tears of lacrymal glands contain a bactericidal protien,called lysozyme,which prevents eye infection.
(ii)Nasal secretions also contain lysozyme capable of inactivating some viruses.
(iii)Saliva of the salivary glands also lysozyme to kill the germs in the food.
(iv)HCL of the gastric juices of the gastric glands of stomach provides a strong acidic medium(1-2 pH) to kill the bacteria in the food.
(v)Certain symbiotic gut micro-organism secrete antibiotics to kill pathogenic bacteria.
(vi)Bile(an alkaline juice with pH 8.0)of liver prevents the growth of bacteria on the chyme in duodenum..
(vii)Certain bacteria of vagina of human female stimulate the production of lactic acid from glycogen of the cells.Lactic acid kills the foreign bacteria.
(viii)Cerumen(ear wax)of ceruminous glands of auditory canal prevents the entry of insects.It also traps dust and bacteria.
              (c)Chemical composition of blood:

Activation of the classical pathway of complement system
(1)Complement system:It isa system of about 20 types of different proteins in the blood plasma.These are generally present in their precursor or inactive form but once activated,these assist or complement body's other defence system.
(2)Antimicrobial substances.Certain animal cells infected with viruses produce polypeptides called Cytokinines.One type of cytokine,an interferon,diffuses to healthy neighbouring cells and stimulates them to produce biochemicals that block viral replication.When these cells become infected,the viruses are unable to take over the protein synthetic machinery to manufacture more of themselves.The spread of the infection halts.
(3)Fever.A fever protects non-specifically.A viral or bacterial infection stimulates white blood cells to proliferate,producing cells that secrete proteins that form a substance called endogenous pyrogen.Endogenous pyrogen resets the thermoregulatory centre in the brain's hypothalamus to maintain a higher body temperature, which may be too high for the infecting microbes to tolerate.

(B) Second line of defence. It is also called macrophage system and is the non-specific internal defence.It is formed of phagocytic cells present within the tissues and the blood stream which ingest and destory foreign particles.It is essential as certain pathogens enter the human body successfully by breaking through the first line of defence.It includes two types of cells:
(i)Leucocytes(W.B.C.s) The polymorphonuclear leucocytes,especially the neutrophils of the blood,have the ability to come out of blood capillaries by amoebiod movements,called diapedesis engulf and digest many of the bacteria that causes acute infection.

Mechanism of phagocytosis
Mechanism of phagocytosis:     
1.Neutrophil first attaches itself to the particles to be phagocytized.
2.It gives out pseudopodia around the particles to form a cup.
3.Pseudopodia fuse and ingest in a food vacuole called phagosome.
4.Phagosome and primary lysosome like proteolytic enzymes and large amount of lipase fuse to form secondary lysosome or digestive granule.These enzymes digest the bacteria.
5.Diffusible material diffuses in cytoplasm while undigested material remains in the phagosome which is now called residual body.It is expelled out of by exocytosis.

(ii)Macrophages.  Macrophages are large amoebiod phagocytic cells which are found in most of connective tissues,lymph nodes,spleen,reticulo-endothelium,liver and bone marrow.Microglia of CNS also act as macrophages.These are able to engulf debries,any colloidal substance,daed cells etc.So these are called scavengers of the body.

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