From time immemorial, man has been interested in trying to control the disease. The medicine man, the priest, the herbalist, and the magician all took in various ways to cure human disease and or bring relief to the sick. Man has an inherent instinct to ward-off diseases and to maintain health from the very first day of life. It is because of this desire that man has developed the science of healing. Society developed progressively, and tremendous changes have also crept-in in the system of healing. Urbanization, globalization, and a free market competitive economy are fast eroding our traditional healthy lifestyle. Man’s natural faculties are becoming commodities of trade and commerce. Human beings overexploit their natural faculties, both mental and physical. When we are supposed to protect ourselves from the outside, we need to protect ourselves beginning right from within our body by strengthening the immune system. The immune system is indeed complex and is to a great extent impacted by the environment around us. Many factors affect the functioning of the immune system.
Immunity is the ability of the body to protect against all types of foreign bodies like bacteria, viruses, toxic substances, etc., which enter the body. Immunity is also known as the disease resistance power of the body.^[1] it is of two types; Innate (natural or nonspecific) and acquired (adaptive or specific immunity). Innate immunity is inherited by individuals from their parents and protects them from birth and throughout life. While the immunity that an individual acquires after birth is called acquired; adaptive immunity protects a person from different infectious diseases and prevents further attacks in the future. In this immunity, a person’s cells produce antibodies in response to any antigen invaded in the body (through clinical/subclinical infection or vaccination); this is called active immunity. When antibodies produced in one body (human or animal) are transferred to another to induce protection against disease, it is known as passive immunity. In other words, the body does not produce its antibodies but depends upon ready-made antibodies.^[2] Here we mean ‘Natural Immunity’ to strengthen and boost up through the interventions and modification in Asbab-e-Sitta Zarooriya (six essentials of life).

Unani Concept of Immunity
In the Unani system of medicine ‘Immunity’, is termed as Tabiyat which may be defined as the total of structural, functional, and psychological characteristics of the human being. In Unani literature, it is believed that Tabiyat itself is a healer and the physician helps it only to cure any ailment and support Tabiyat for the same. Tabiyat is a supreme planner of the body to build a healthy environment within the body and prepare it to fight against the disease. If Tabiyat is strong, then a person does not endure a disease easily; if it is weakened, then a person becomes prone to disease easily.
According to Hippocrates, there is a special capability concealed in every individual called the defensive mechanism of the body or ‘Tabiyat’, and this Tabiyat is the best physician who maintains the equilibrium of four body humours (blood, phlegm, yellow bile, and black bile). For the upholding of health, the quality and quantity of these humour should be as per the natural chemical composition of the body. Broadly speaking Tabiyat is considered the real healer of the body and the role of the Tabeeb (physician) is to assist the Tabiyat.^[3]
Razi (865AD-925AD) thought that Tabiyat is exclusively responsible for the performance of all physiological functions (Afale-Tabiyah) whether it is a nutritional requirement of the body or substitution of what is lost after Istehala (Metabolism); it excretes the waste products. Another very important function of the Tabiyat is the translocation of harmful matter from vital organs to the lesser organs or outside of the body and alteration of temperament according to the internal environmental demand. It capacitates the body to defend against diseases and pathogens.^[4]
Rabban Tabri (775-890AD) in his treatise, ‘Firdousal Hikmat’ has mentioned about Tabiyat that practically Tabiyat is regarded as an administrative power of the body. The function of administration of the body is accomplished by Tabiyat with the help of various powers called Quwwa of the body like Quwwate tabaiyya (Physical faculty), Quwwate haiwania (Vital faculty), and Quwwate nafsania (Nervous faculty). Thus the main role of the Tabiyat is to provide the general administration as well as defense or immunity to the body.^[5],^[6]
Ibn-e-Sina (980AD-1030AD) assumed in ‘Al-Qanoon-Fil Tib’ that the Tabiyat-e-Insaniah (Human Physic) provides the capacity to resist the diseases and to make a defensive line against the Madda-e-Marz (Disease causing matter). When the human body is exposed to Madda-e-Marz, Tabiyat takes on it until it overcomes the disease or becomes perplexed itself. In the last stage, combat between Tabiyat and Madda-e-Marz gets intensified and domination of one other appears.^[7]

Asbab-e-Sitta Zarooriya and Immunity (Tabiyat)
In the Unani system of medicine, Tabiyat is an individual’s internal power or capacity to withstand or combat disease or perform normal physiological functions. Strengthening tabiyat can better be obtained with moderation in Asbab-e-Sitta Zarooriya. Unani medicine recognizes six factors called as Asbab-e-Sitta Zarooriya which are essential in establishing a synchronized biological system and thus living a balanced existence. These six factors are Hawa-e-Muhit (Atmospheric Air); Makool-wa-Mashroob (Foods and Drinks); Harkat-wa-Sakoon-e-Badani (Physical activity and Rest); Harkat-wa-Sakoon-e-Nafsani (Psychological activity and Repose); Naum-wa-Yaqzah (Sleep and Wakefulness); Istifragh-wa-Ihtibas (Elimination and Retention)

Hawa-e-Muheet (Atmospheric Air)
Air has got priority over all the six essential factors, without air we cannot imagine the existence of life. Regularly breathing fresh air is an effective way to help strengthen the immune system and fight off pathogens.^[8] The quality of the air, a person breathes is thought to have a direct effect on his/her temperament and thus health. Pure and clean air is a must for better health; many diseases occur due to changes in the air. The seasonal changes in the air support favour the Tabiyat and are beneficial to health. But sometimes these changes are unfavorable to Tabiyat and result in sue mizaj (mal-temperament) of ruh (Psyche/Soul) and qalb (heart) and are responsible for ill health. E.g. wabai hawa (epidemic air).^[9]
Waba (epidemic) is believed to be an alteration in the ‘jauhar’ (essence) of air. As a result, the air becomes impure and finally, it leads to the mal temperament of ruh which may result in the morbidity or mortality of a large number of people.^[9] This situation persists until the air becomes pure. As soon as the air becomes normal in its characteristics, the affected individuals gain their normal temperament and their health is restored. Therefore, a direct relationship between air and temperament exists. Any contagion in the air may affect the healthy life either slowly or suddenly depending upon the intensity of contamination. If the atmospheric air contamination is abrupt and intense, it may result in sudden death due to suffocation. The chronic effects related to prolonged inhalation of contaminated air e.g. chronic bronchitis, bronchial asthma, respiratory allergies, emphysema, and lung cancer.^[10]
Ibne Sina (980-1030AD) in his famous treatise, ‘The Canon of Medicine’ states that the change in environment relieves the patients of many diseases. He has also emphasized the need for open, airy houses with proper ventilation, playgrounds, and gardens in the cities so that everyone has access to fresh air and a proper ecological balance is also maintained.^[11]

Makool-wa-Mashroob (Food and drink)
Balanced and good quality food is essential for the promotion of health while any imbalance either in quality or quantity may lead to a variety of diseases. The nutritional value of an individual’s diet in terms of quality and quantity of food and drink is believed to ensure fitness by strengthening Tabiyat. The most important element that maintains our immune system is a healthy balanced diet, which contains all the nutrients in terms of carbohydrates, proteins, fats, vitamins, and minerals in a balanced proportion. The adaption of healthy eating habits such as the addition of fresh vegetables and fruits of all kinds in large quantity plays a pivotal role because they contain many useful and valuable nutrients, nuts having proteins and magnesium, fish because they contain zinc which works to produce blood cells that fight infection, mushrooms, garlic yogurt, fiber, and olive oil having high-quality antioxidants.^[12],^[13],^[14],^[15]
For the healthy living of an individual, the significant role of food and drink and selection of a good diet for the prevention and treatment of diseases was quoted by many ancient Unani physicians. Like Hippocrates stated ‘the cause of sickness is overeating and the cause of health is eating like a bird’, ‘take diet only when you have desire’, and ‘good wines in small quantity is the friend of body and apple is the friend of the soul’.^[16] Another statement of Hippocrates (460-370BC) is ‘let your food be your medicine, and medicine be your food’, and ‘leave your drugs in the chemist’s pot, if you can heal the patient with food’.^[17] Pythagoras (570-495BC) acknowledged that ‘people should take care of their health; the diet, coitus, and exercise should be in a balanced way’15 whereas Razi said that ‘good nutrition, adequate rest, happiness and best line of treatment are the pillars for curing of diseases’. He also stated that ‘the amount of desired food items should be less for a patient’, and ‘whenever possible, treatment of the diseases should be done by diets only not by drugs’.^[18] Haris bin Kalda (570-633AD) quoted that ‘pomegranate is the best fruit among all the fruits, rose is the best essence among all the essences, and kasni (Cichorium intybus) is the best vegetable among all the vegetables’.^[16] Jalinoos suggested that four conditions should be kept in mind while making eating or drinking habits^[19] Time of the food; Type of the food; Quantity of the food; The temperament of the food.

Recommendations of Diet for adults and elders
In the context of beverages and simple water Unani physicians said that don’t use it after a meal as indigestion and other gastric problems take place. While Galen said, ‘We should take first soft food and then hard foods’. Razi thought, ‘ Anyone who wants to be healthy, so he/she should take the same type of diet in small quantity’. Ali Bin Rabban Tabri suggested that ‘One should take the choice of foods at the time of full hunger and should take rest after taking meal’. The old aged persons are also prone to illnesses as innate energy, immune system and faculty of nutrition become weak. They should take nutritive diets but in little amounts frequently. It is also advised; don’t take heavy fatty diets like red meats and Ghiza-e-Barid (cold diets) like curd and cold beverages etc. (Park, 2011/ Razi, 1991). Jalinoos (Galen) affirmed that elderly people should eat food thrice a day.
General Dietary Recommendations for better Health include a suitable diet should be chosen; Regularity and punctuality of diet are essential; Overeating should be avoided; During indigestion, the diet should be reduced and better to avoid it; the Blessed Messenger of Islam stated that ‘stomach should be filled 1/3rd by food, 1/3rd by water and leave 1/3rd empty for breathing’. Diet should be taken on genuine hunger, unless the previous meal is digested, but should not delay the meal until the appetite has passed off; It is harmful to the body to superimpose food over a previous diet; Mother’s milk is the best diet for the infant; A balanced diet in terms of quality and quantity is the key to maintaining Taghzia (Nutrition); Diet should be according to Season: During winter, hot food and summer, cold food should be preferred; In winter more nutritious and solid food like cereals should be taken and poor nutritious food like vegetables should be avoided; The best food for everyone is that which grows where they live; Fruits should be taken before a meal.^[20]-^[26]
Drinking water in various formats can be beneficial for different aspects of health. The importance of drinking water cannot be emphasized enough given its endless benefits to immunity. Water is most important in all liquid substances because it is a vehicle for food materials by which the food becomes thinner and reaches the organs.^[27] Ibn Sina stated that ‘Water is that essential element which produces flow in food substances and liquefies it’. Water helps to carry oxygen to our body cells, which results in properly functioning systems. It also works in removing toxins from the body, so drinking more of it could help prevent toxins from building up and retaining in the body having a negative impact on the immune system.^[28] Drinking water immediately after meals, should be avoided because it hinders with the digestive process. It must be taken after descending foods from the upper digestive tracts or on severe thirst; this may prove beneficial to the digestive process.^[29] Hippocrates forbade water intake during the midnight hours because it causes failing of innate energy which leads to indigestion. Excessive cold water intake creates problems for the nervous and respiratory systems, particularly in childhood and older age. Avicenna advocated that intake of warm water also causes improper digestion, and vomiting and occasionally becomes a leading cause of ascites and as well as weakness. But he advised for intake of lukewarm water because it is beneficial to take away constipation and relaxes the rectal muscles to release bowel easily but don’t take in excess amount because it hampers digestion. (Tabri, 2002/ Maseehi, 2008/Kirmani, 1934) Ibn-e-Nafis (1210-1288AD) corroborated that on empty stomach one should refrain from intake of water as it causes indigestion, he also suggested the same suggestion i.e. away from consumption of water after riyazat (exercise especially heavy exercise), Jima (sexual intercourse), Hammam (hot bath), after taking Qavi Mushil (heavy purgatives), eating of dry fruits and after taking of Tarbuz (watermelon), cucumber and Kharbuz (sweat melon), etc. People having Har Mizaj (hot temperament) should consume Barid Mashrubat (cold liquids/Juices) like orange, apple, pomegranate, pineapple, lemon juices, etc. (Kirmani, 1934/ Razi, 2000) Drink extra amounts of water throughout the day by a glass of water 30 minutes before eating helps digestion, a glass of water before the bath helps to lower blood pressure, a glass of water before sleep prevents heart attack. It is presumed that increased drinking water works on the sedimentation of mucous membranes lining the respiratory tract, which helps the production of antibodies and white blood cells, thereby strengthening the immune system’s performance.^[30],^[31] The average daily per capita consumption of drinking water is usually found to be around 2 liters, but there are considerable variations between individuals as water intake is likely to vary with climate, physical activity, and culture, e.g., at a temperature above 25°C, there is a sharp rise in fluid intake, largely to meet the demands of an increased sweat rate.^[2]

Harkat-wa-Sakoon-e-Badani (Physical activity and Rest)
It emphasizes the positive effects of balanced physical exercise on an individual’s internal resistance and tabiyat. The health benefits of regular exercise and physical activity are hard to ignore because it is one of the strong pillars of healthy living. It improves and strengthens cardiovascular health, lowers blood pressure, burns calories while building a stronger body and helps in controlling the body weight, and protects against a variety of diseases. Just like a healthy diet, exercise can contribute to general good health and therefore to a healthy immune system. It may contribute even more directly by promoting good circulation, which allows the cells and substances of the immune system to move through the body freely and do their job efficiently.^[32]
Riyazat (Exercise) helps to boost the innate heat of the body; a unique tool of Tabiyat, which then overcomes disease-causing matter, and thus helps in fighting against the diseases. Unani physicians advocated that physical activity is essential for the activation of hararat-e-ghariziya (innate energy) and to expel the waste products of the body through channels of excretion when the person follows the rules and regulations with reference to types, time, intensity, and occupation. Razi (865AD-925AD) has described types, time, uses, and precautionary measures before and after riyazat in ‘Kitabul Murshid’. Avicenna (980 AD-1030 AD) has well discussed riyazat in ‘Al Qanoon Fi Tib’ and elaborated on the mechanism of actions, methods, varieties, special exercise for each organ, therapeutic exercise, the limit and amount of riyazat.^[33]
Types of Riyazat include Riyazate Kulli (Complete exercise) e.g. horse riding.^[34],^[35] Riyazate Juziya (Partial exercise) e.g. stone lifting.^[34]-^[37], Other Types of Riyazat include Riyazat-e-Ararziya/Ghair Iradi (unwillingly): Exercise in which there is no will to do exercise, it is done in daily ordinary occupational activities like for iron-smith and washer man, etc.34-36, Riyazat-e-Zatiya Khalisa / (willingly): This exercise is done purposefully to gain its benefits. It is further subdivided according to its duration, strength, mode, etc.34,36, Types of Riyazat-e-Zatiya/Khalisa according to duration34,38 include Riyazat-e-qaleela (short duration exercise), Riyazat-e-kaseera (long duration exercise), Riyazat-e-moatadil (moderate duration exercise). According to strength^[34]-^[38] include Riyazat-e-qawiya/shadeeda (forceful exercise), and Riyazat-e- zaeefa (mild exercise in which less force is used). Riyazat-e-moatadil-average strenuous exercise in which average force is used. According to sura’at (Frequency)^[34],^[38] include Riyazat-e-sariya-the exercise in which movements should be rapid and fast, Riyazat-e-batiya-exercise in which movements should be dull, and delayed, Riyazat-e-moatadil-movements in between riyazat sariya and batiya. According to strength and sura’at^[38] include Riyazat-e-hasheesha-strong and fast movements are done, Riyazat-e-mutarakhiya-in which movements are weak and slow, Riyazat-e-moatadil-exercise between hasheesha and mutarakhiya.

Recommendations of riyazat
Exercise should be according to age, temperament, occupation, body conditions, time, etc. The best time for exercise is during the moderate condition of the body.34 Before exercise, the intestine, and urinary bladder should be empty.^[34] Dalake istedad (Massage to warm-up body) should be done before exercise and Dalake isterdad (massage for relaxing body) after exercise.^[34],^[36],^[39] It should be done after complete digestion of food.^[34],^[35],^[40] During heavy exercise there should be some Lateef (light diet) in the stomach, especially in the summer season, and in winter there should be some Ghaleez (heavy diet) in the stomach. Exercise should be done in the morning hours.34,36 On an empty and full stomach, exercise should be avoided.^[34],^[36],^[39],^[40] If there is excess Hararat (hot), Yubusat (dry), and Burudat (cold) in the body, exercise should not be done.^[34],^[40]
Timing of riyazat ^[34],^[36]
In Rabi (spring season) good time for exercise is noon, In Saif (summer) exercise should be done in the morning, In Shita (winter) exercise should be performed in the evening.
Quantity of riyazat
It should stop gradually and three things should be kept in mind. Colour of the body: Exercise should be done till the color of the skin remains shining, and if the color is going to dull or yellow, then exercise should be stopped.^[34] Movements (harkat) of the body: When the movement of the body is being performed easily exercise should be continued. If there is a feeling of fatigue it should be stopped.^[34],^[36] Swelling of the organs (aaza ka phoolna): Exercise should be continued till the organs are swollen and sweating is dried up and when the organs stop swelling and sweating continues, exercise should be stopped.^[34],^[36]
Rest is essential to relieve fatigue and to decrease the body temperature which is harmful to body fluids. Excess of both causes coldness of the body because motion leads to the decrease of the innate fluids, excess of rest increases the body fluid which decreases the innate energy.^[41] hence, and there should be balance in physical activity and rest. Ibn-e-Sina said that ‘exercise is the cause of good health if it is done at the right time and in moderate quantity’.^[42],^[43]

Harkat-wa-Sakoon-e-Nafsani (Psychological activity and Repose)
It emphasizes the simultaneous engagement of the human mind in several intellectual and emotional activities. Just as the body needs systematic and planned exercise and rest, Unani medicine holds that the human mind and brain need adequate stimulation and proper relaxation.
Psychological factors such as happiness, sorrow, fear, anger, etc. have a significant bearing on the health of a human being. Excess anger and excessive joy cause cutaneous blood vessels to dilate which provides a reddish hue to the skin. This also increases the heart rate which becomes the cause of various cardiovascular diseases in later life. These factors are discussed in Unani classical texts under the title of ‘Amraaz-e-Nafsani’ (psychiatric disorders) like Melancholy hysteria and depression. Depression is mentioned as a symptom or group of symptoms of Malankholia (Melancholia) in which the mental functions of the individual are deranged leading to constant grief, fear, and dubious aggression. The patient finds himself/herself deserted and occupied by loneliness and some patients experience delusion and hallucination.^[44],^[45]
When someone is under stress, the immune system’s ability to fight off infection is reduced and the person becomes more susceptible to infections. Corticosteroids, a stress hormone can suppress the efficacy of the immune system by lowering the number of lymphocytes. Stress can also have an indirect effect on the immune system as a person may use unhealthy behavioral coping strategies to reduce their stress, such as drinking and smoking. Stress is linked to headaches, infectious illness (e.g. flu), diabetes, asthma, and gastric ulcers. Stress responses have an effect on the digestive system. During stress, digestion is inhibited and when the stress is relieved, digestive activity increases. This may affect the functioning of the digestive system. Adrenaline released during a stress response may cause ulcers. Stress responses increase strain on the circulatory system due to increased heart rate etc. Stress can also affect the immune system by raising blood pressure.^[46] Equilibrium of both mental activity and rest is required for the preservation of good health and to avoid many physical illnesses. As the famous quote says, ‘Sound mind in a sound body’.^[7],^[42],^[47],^[48]
To overcome these maladies Unani system of medicine involves Nafsiyati Tadabeer (Psychological therapy) like distracting the mind through adaptive changes and through various other methods which make the patients pleased and happy. Adaptive changes are, living in a clean, well-lit house, which has provisions for fresh breezes of air, wearing white or pastel shades, ensuring comfort, and entrusting with responsibility. Distracting methods include visiting hill stations and parks, viewing plays, hearing pleasant music, being in the company of religious people, listening to religious sermons and interesting stories, reading books and inculcating hobbies, etc.^[49],^[50],^[51]

Naum-wa-Yaqzah (Sleep and wakefulness)
Normal sleep and wakefulness are essential for health. The advantage of sleep is that, it provides rest to the nafs (Psyche) and organs which makes a person more active and it also helps in digestion which makes person healthier. Sleep plays an important role to repair and regenerate the body function. Healthy persons should be particularly careful about sleep, it should be regular moderate, and not too long. Every individual needs proper 7 to 8 hours of sleep to repair body functions. Alteration in sleeping patterns affects the body’s immune function, hormone secretion, and mental and physical stamina.
Sleep and wakefulness are essential factors of life. Sleep is analogous to rest and wakefulness to movement. The normal physical and psychosocial functions depend on adequate sleep.7 An eminent Unani scholar Zakariya Razi delineates that eight hours of sleep is very essential for health. He also says that sleep strengthens the vital faculty, pneuma, and promotes digestion8 by retaining hararat-e-ghareeziyah.^[52],^[53] Movements cause the formation of hararat (innate heat) and help in removing the waste products by disintegrating them and also preparing food for combustion. Therefore, sleep is necessary before any movement so that body can get nourishment. Majoosi said that tabiyat is benefitted in two ways by sleep; one is mental and physical rest and the second is the digestion and concoction of akhlaat (body fluids) which causes innate heat to enter the body.^[23],^[48]

Istefragh-wa-Ihtebas (Elimination and Retention)
Istifragh means the elimination of unnecessary substances out of the body because retention of these substances inside the body produces disease e.g., constipation.^[54] In the same way, elimination of the substances that must be retained causes the abnormal condition.^[55] Elimination is carried through normal channels of the body such as the passage of sweat, urine, stools, menstrual blood, etc; but excess excretion results in an abnormal condition.^[47] The excess loss of fluid, salts, and nutrients in the stools, causes abnormalities. The excessive elimination of any matter always causes the coldness and dryness of temperament directly^[55] and decreases the innate energy. If elimination and retention are balanced and take place at the time when they are needed; they are beneficial and maintain health.
It is said that up to 80% of the immune system is in the gut making it the largest immune organ in the body. Constipation slows down the peristalsis movement within the intestines, which means that food matter and waste get stuck preventing good absorption of key nutrients; some of which are needed to support the immune system as well as other side effects such as inflammation and bloating. Over time, this lack of nutrient absorption and build-up of food matter will weaken the immune system making the person more susceptible to the viruses that cause coughs and colds.
Constipation also affects the balance of good bacteria within the digestive system which is crucial for the strength of the immune system. Good bacteria have a major role in the breakdown and digestion of food. They also play a decisive role in being a part of the immune response and produce substances that drive off or kill invading pathogens (such as the flu virus). So, if good bacteria are compromised as a result of constipation the person is more likely to fall ill.^[56] Hence, disturbances in bowel function due to chronic constipation may result in changes in the colonic flora which may lead to disordered immunity and decreased resistance to pathogenic flora.

Conclusion
The main role of the Tabiyat is to provide the general administration as well as defense or immunity to the body. The disease is the consequence of the defeat of tabiyat because when ṭabiyat is strong, it can easily combat the causative matter and maintain normalcy. Tabiyat has a direct or indirect relationship with the six essential prerequisites in the restoration of health. Unani medicine strives to find the best possible ways by which a person can lead a healthy life and bare minimum of sickness. Unani scholars assume that diseases can be avoided by enhanced immunity through moderate physical exercise, improving sleep quality, better breathing methods, using clean and fresh air, and consuming fresh food and safe wholesome water. Likewise, a balance should be maintained between the mind and the body so that the metabolic process can take place easily and proper evacuation of body wastes. From the above discussion, it can be concluded that upgrading immunity through the moderation of the six essential factors of life (Asbab-e-Sitta Zarooriya) maintains and promotes positive health as well as prevents the development of diseases.

Conflict of Interest
The author declares that there are no conflicts of interest relevant to this article.