Introduction:

The immune system is basically the defense of body against the infectious organisms and other invaders. Via a series of steps termed as the immune response, the immune system attacks organisms and substances which invade body systems and cause disease.

The immune system is build up of a network of cells, tissues and organs which work altogether to protect the body. The cells comprised are white blood cells or leukocytes that come in two fundamental kinds that join to search for and destroy disease-causing organisms or substances.

Leukocytes are generated or stored in numerous locations in the body, comprising the thymus, spleen and bone marrow. Due to this reason, they are termed as the lymphoid organs. There are as well clumps of lymphoid tissue all through the body, principally as lymph nodes, that house the leukocytes.

The leukocytes circulate via the body among the organs and nodes through lymphatic vessels and blood vessels. In this manner, the immune system works in a coordinated way to monitor the body for germs or substances which might cause problems.

Infection:

An infection is the colonization of the host organism through parasite species. Infecting parasites look for to employ the host's resources to reproduce, often resultant in disease. Colloquially, infections are generally considered to be caused due to microscopic organisms or micro parasites such as prions, viruses, bacteria and viroids, although bigger organisms such as macro-parasites and fungi can as well infect. 

The hosts generally fight infections themselves through their immune system. Mammalian hosts react to infections by an innate response, frequently comprising inflammation, followed through an adaptive response. Pharmaceuticals as well assist in fighting infections.

The diagnosis of infections can be hard as particular signs and symptoms are rare. When an infection is suspected, urine, blood and sputum culture tests are generally the first step. Chest x-rays and stool analysis might as well help in diagnosis. Spinal fluid can be tested to make sure that there is no brain infection.

In case of children, the presence of cyanosis, fast breathing, poor peripheral perfusion or a petechial rash raises the risk of a serious infection by greater than 5 fold. Other significant indicators comprise parental concern, clinical instinct and temperature more than 40 °C.  

Infections could be as an outcome of virus or bacteria. Bacterial and viral infections can both cause symptoms like malaise, fever and chills. It can be hard to differentiate among bacterial and viral infection however it is significant to do so, as viral infections can't be cured by the antibiotics.

Infection starts if an organism successfully colonizes a host by entering the body, growing and multiplying. Most of the humans are not simply infected however those who are sick, weak and malnourished have cancer or are diabetic have raised the susceptibility to chronic or persistent infections. Individuals who encompass a suppressed immune system are specifically susceptible to opportunistic infections. Invasion of the host usually takes place via the mucosa in orifices such as the oral cavity, eyes, nose, genitalia, anus or open wounds. As some organisms can grow at the initial site of entry, lots of migrate and cause systemic infection in various organs. A few pathogens grow in the host cells (that is, intracellular) while others grow freely in the bodily fluids.

Infections yield in diseases. Disease can occur when the host's protective immune methods are compromised and the organism imposes damage on the host. Microorganisms can cause tissue damage by discharging a diversity of toxins or destructive enzymes.

Examples of Infections:

1) Specific bacterial infections:

H pylori are related by means of inflammation of the stomach and are a general cause of stomach ulcers and gastritis. At least 10 % of individuals infected by H pylori grow an ulcer. Though, there is a raised risk of stomach cancer after an infection by this organism.

Methicillin-resistant staphylococcus aureus mainly influences the skin and is considered to be a super bug as it is much resistant to antibiotics. This bacterium is termed to produce a diversity of toxic enzymes that can lead to diarrhoea, vomiting, shock and sepsis.

MRSA is pretty common in hospitals and nowadays there is great cause for concern regarding its spread. 

2) Chronic ear infections:

Chronic ear infections are a general problem in childhood. These infections might be because of bacteria or the common cold virus. The disorder frequently exhibits by persistent blockage of the ear, chronic ear drainage, hearing loss, balance problems, headache, deep ear pain, fever, surplus sleepiness or confusion. Chronic ear infections generally develop slowly over numerous years in patients who have had ear problems. The cure of persistent ear infections is complex and needs a combination of corticosteroids, antibiotics, and/or placement of tubes.

3) Osteomyelitis:

Osteomyelitis is a bone infection mainly caused due to a variety of bacteria and can take place in both adults and children. When bone gets infected, there is constant pain, fever and it is painful to move the edge. This kind of infections is acquired from infections elsewhere in the body, from trauma or are spread from nearby infected tissues. The analysis of bone infection needs a bone scan, blood cultures and x-rays.

At times the bone marrow is aspirated to discover the particular organism. Osteomyelitis is a serious infection and carries a high complication rate when not treated on time. If the infection is diagnosed quickly, the prognosis is excellent. Though chronic Osteomyelitis can take years to heal and can keep on recurring. Individuals at the risk of Osteomyelitis comprise those who encompass artificial joints or metal parts in their joint.  

4) Lyme disease:

Lyme is a tick borne disease which can cause a skin rash, chills, fever, body aches and joint pain. A few infected individuals build up severe weakness and temporary paralysis. Lyme disease is caused due to at least three species of bacteria belonging to the genus Borrelia that is taken out by deer ticks. Infections are more common throughout summer, particularly when the host spends time in grassy woodlands where ticks breed. If the infection is diagnosed promptly, most of the people do recover completely. Though, there are a few individuals who keep on having recurring or lingering symptoms long subsequent to the infection has been treated. If it becomes chronic, Lyme disease can present by a diversity of symptoms comprising migrating joint pains, headaches, excess fatigue, confusion, incapability to sleep, paralysis of one side of the face and difficulty concentrating. Even although there are reliable tests available they are not 100% sensitive. As most of the individuals do respond to a 14 day course of antibiotics, a few individuals take considerably longer.

Prevention as a Means of Protection:

Viable treatment and prevention policies will disturb the infection cycle. For illustration, direct transmission can be reduced by adequate hygiene, sustaining a sanitary environment and health education.

If infection attacks the body, anti-infective drugs can repress the infection. Four kinds of anti-infective or drugs exist: antibacterial (antibiotic), antiviral, antitubercular and antifungal. Based on the severity and the kind of infection, the antibiotic might be given by mouth, injection or might be applied topically. Some infections of the brain are generally treated by intravenous antibiotics. At times, multiple antibiotics are employed to reduce the risk of resistance and raise efficiency. Antibiotics just work for bacteria and don't influence viruses.

Antibiotics work by slowing down the multiplication of bacteria or through killing the bacteria. The most general classes of antibiotics employed in medicine comprise penicillin, aminoglycosides, cephalosporins, quinolones, macrolides and tetracyclines. 

Methods such as hand washing, wearing gowns and wearing face masks can assist in preventing infections from being passed from the surgeon to the patient or vice-versa. Frequent hand washing remains the most significant defense against the spread of unwanted organisms. Nutrition has to be enhanced and one has to make modifications in life style-like avoiding the utilization of illicit drugs, utilizing a condom and entering an exercise program. Cooking foods fine and ignoring eating foods which have been left outside for a long time is as well significant. Don't take antibiotics for longer than required. Long term utilization of antibiotics leads to the resistance and chances of developing opportunistic infections such as clostridium difficile colitis. Vaccination is the other means of preventing infections through facilitating the growth of immune resistance in the vaccinated hosts.

Non-Specific Resistance:

Innate or non-specific resistance is the method through which the host removes most invading micro-organism through non-specific immunological means. There are three main methods of non-specific immunity to infection:

a) Surface or Mechanism barriers

b) Action of phagocytic cells

c) The role of complement

Surface/Mechanism Barriers:

The skin is the most evident mechanical barrier preventing micro-organisms from growing the access to the body. Fatty acids and Lysosome included in sweat lower the pH and render the skin uninhabitable to most of the bacteria or immobilize the micro-organisms. The mucociliary action of cells lining the respiratory area is the other non-specific mechanical protective method, as is the acid generated in the stomach that renders the gastrointestinal environment hostile to the infectious agents. The mucociliary lining of the lungs, saliva and tears all give some protection against infection.

Action of Phagocytic Cells:

Once the micro-organisms penetrate the surface barriers, it becomes exposed to the action of phagocytic cells. Phagocytosis of micro-organisms and their killing intracellular through prteolytic and other enzymes is the most primitive of defenses against microbes that manage to gain access into the body. The three main cells comprised in non specific immune responses to infection comprise; macrophages, polymorphonuclear neutrophilis (PMN) and eosinophils.

Specific Immune Response to Infection:

Specific humoral immunity is mainly based on the antibodies directed against the invading micro-organism. There are two major stages in antibody production. After initial contact by the antigen a few days or weeks elapse before antibodies are generated. Initially IgM then IgG antibodies are generated however the titres of both are low in this phase, termed as the primary antibody response. Later exposure to the similar antigen evokes a much faster and more sustained antibody production. This secondary response resultant in a persistence of the antibody is responsible for resistance to infection. Antibodies can as well directly neutralize the effects of micro-organisms by joining with their toxins, therefore preventing the appearance of illness. Antibodies as well act in concert with complement to cause the bacterial lysis. Antibody-based parasite killing is best exemplified through schistosome in a cyotoxic way that is independent of complement. 

Cell-mediated Immunity:

This significant in those infections where the organisms are intracellular as they are inaccessible to the humoral antibody. Particular cell-mediated immunity to infection is the function mostly of T lymphocytes and macrophages. Contact among antigen, macrophages and lymphocytes leads to the making of lymphokines, memory T lymphocytes and killer cells. These killer cells attack and demolish infected cells bearing the antigen in question discharging the microbes in them which now become depicted to humoral antibody able of neutralizing them or their consequences. The lymphocyte killing of the target cell doesn't need antibody or complement. Lymphokines are soluble factors discharged by T lymphocytes as an outcome of contact by antigen. The lymphokines improve the phagocytic action of macrophages. Macrophages are activities by means of the lymphokines. Theses activated macrophages show greatly improved cytotoxicity to infectious agents and also tumor cells. Macrophage activation is significant in regulating infections like leprosy, tuberculosis and a number of the viral infections.

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