The immune′ sys`tem definition: n.
A diffuse, complex network of interacting cells, cell products, and cell-forming tissues that protects the body from pathogens and other foreign substances, destroys infected and malignant cells, and removes cellular debris.

Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. 

What is the immune system and how does it relate to our health?  To help you fully understand the immune power within you, let’s simplify what it is and how the immune system works to protect you.

The immune system is made up of 2 parts: the immune system as a whole and the lymphatic system.  Both work together to protect you from illness and disease.

The lymph fluid is a clear fluid that carries white blood cells through the lymph vessels all around your body to help fight infection and disease. There is no pump for this system, like the heart is to the circulatory system.  The lymph fluid bathes, nourishes and brings nutrients to your cells as it also collects and carries away wastes, microbes and toxins. These toxins are carried to the lymph nodes that act as filters. They are located in the neck, armpits, groin, chest, abdomen and all around the body. They often swell when they’re actively fighting infection.  All the components of your immune system act synergistically to protect your body from invasion. There are 3 lines of defense.

The first line of defense is:
-The skin
-The cornea of the eyes and tears
-Membranes lining the respiratory, digestive, urinary, and reproductive tracts and the secretions they produce. As long as these barriers remain unbroken, many invaders cannot enter the body. If a barrier is broken—for example, if extensive burns damage the skin—the risk of infection is increased.

The second line of defense are our white blood cells. There are several different types that do different things. There are also certain molecules that attract immune system cells to affected tissue. 

The third line of defense are the organs that participate in our immune system. 

-The bone marrow
-the thymus
-the tonsils
-the lymph nodes
-Peyer patches in small intestine
-the appendix 

These organs trap microorganisms and other foreign substances and provide a place for mature cells of the immune system to collect, interact with each other and with foreign substances, and generate a specific immune response.  All of these complex components provide our bodies with a miraculous system of defense to ward off sickness and disease throughout our lives. The immune system is constantly working without our even thinking about it (until we get sick!).

Yoga helps us realize that not only the physical toxins that we encounter day to day in our environment affect us, but also by the pollutants to our mind, our spirit, our energy  and our divine nature. The practice of yoga is an organic, holistic  immune system "booster" that provides us with natural ways to reduce the effects of not only physical, but also mental, emotional, and spiritual pollution.  There are countless studies that show that yoga improves immune functioning in all layers of our being and  it is actually working to benefit us on the genetic level through the spiritual level.

Consider: if your body is healthy but your mind is filled with negativity, stress or toxic thoughts, you cannot function optimally. This can result in dis-ease. If your mind is positive, quick and bright but your body is ill, you are incapable of giving your all. This can result in dis-ease. If your mind and body are quite healthy but your spirit is in pain from loss, or trauma it is difficult to live wholly, openly and with trust. This can result in dis-ease. 

Yoga focuses on the purification of body, mind and spirit to move us towards enlightenment and encourages more mindful and healthy behaviors to aid us on this path.
With a steady and regular yoga practice, we improve physical strength and stamina, and aid in the circulation of lymph fluid and the oxygenation and circulation of blood. Yoga asana, especially inversions, are very helpful in moving the lymph fluid throughout the body and stimulates the various organs and systems to boost their functioning and increase their efficiency. Yoga gives us a much greater sense of  body awareness which enables us to detect sooner when something is not right.  By being more in touch with ourselves we're able to administer self care more quickly and efficiently by adjusting our asana or breathing practice to a more gentler one, spending more time in meditation, eating more mindfully, and resting when necessary.  
To calm and control the mind, we learn that meditation and the practice of positive thinking are highly effective tools to stop the 'monkey mind'. Again, developing a steady meditation practice for a short time every day will change your life. Taking charge of a runaway negative mind by replacing negative thoughts with positive ones is a way to retrain the mind to serve us in a healthy way rather than letting the mind control us to the point of depression, anxiety, or anger and fear based behavior.  The use of visualization is also very helpful in boosting our mental immunity. 
Yoga helps us cope with spiritual emptiness or feelings of loneliness or hopelessness by creating community, and encouraging the practice of Karma Yoga or selfless service to feel connected and give life meaning.  It also helps us to develop healthier internal coping mechanisms and strategies for approaching challenging situations, and cultivate a compassionate relationships with ourself and others.  Ultimately, through the path of yoga, we come to an  understanding that we are each an integral piece of a huge puzzle, each of us having important gifts to give and parts to play,  and that there is a higher power that will always support and nourish us in all ways. The meaning of yoga is union, not only the union of our own mind body and spirit, but the union of all human beings and all life and the profoundly intimate connection that we all share. When we really come to see this, we are immune to anything that can potentially harm us because we realize our eternal, infinite nature goes on forever. 

With health, joy and ease,