Health is defined as a state of homeostasis within the body, human or animal.  It is a state in which all cells are normal, performing their daily function and receiving the necessary nutrients to perform efficiently.  A state of homeostasis also implies that the body is free of disease.
     Disease is defined as a state of abnormality within the body that may be affecting one or multiple organ systems.  The disease may be acquired via natural mechanisms or through an infectious etiology.  Disease may also be inherited, in terms of a genetic defect of cellular function or anatomy.
     Many of today's diseases have been shown to have a close connection with chronic inflammation within the body.  When you plainly analyze the terminology used to label various diseases, it is common to note that many names end in the suffix "itis".  This suffix plainly relates or implies inflammation.  For example, we may label a disease such as pancreatitis, colitis or even tendonitis.  The funny thing is that we have noted inflammation to present in a multitude of disease processes, but only recently are researchers actually making a connection between one and the other.  The process of inflammation is characterized by redness, heat, swelling and pain.  This holds true in the simple case of blunt trauma, such as hitting your thumb with a hammer.  In this instance, tissue is traumatized, blood vessels are damaged and begin to leak fluid into the surrounding tissue.  The cells in the local area that are damaged being to secrete various proteins called cytokines that then trigger other cells from the immune system to come to the area.  The blood circulation to the area is increased, delivering various nutrients and allowing other cells access to the area.  As more cells are recruited, more cytokines are released into the general circulation.  These various cytokines and eicosanoids are the topic of discussion and focus of research on the human side.
     When we think of inflammation, we generally think of instances such as the hammer to the thumb or a twisted ankle.  This is certainly true, but all living organisms undergo inflammation at a cellular level on a daily basis without external trauma.  Cells are dying on a daily basis, releasing cytokines into the bloodstream.  Other concurrent conditions such as obesity, diabetes, arthritis, allergies, asthma and even UV radiation elicit inflammation within the body, not to mention various habits such as tobacco use or alcohol consumption.  All of these incitors lead to cellular changes and eventual cytokine release.  Stress is also a major contributor to inflammation in the body on a daily basis.  Various foods that we consume also contribute to inflammation.  Various irritants or carcinogens are found in our food and in our environment that also lead to an inflammatory response.
     Cytokines released in our body or general circulation lead to various changes at a cellular level as well as stimulating further release of other potentially damaging substances such as prostaglandins, leukotrienes and reactive oxygen species (ROS).  Now, all of these things are important and a natural response to inflammation.  It is a necessary process to help protect our body and stimulate healing, the problem comes when the process gets out of control or persists for an extended period of time.  One of the major changes that occurs with chronic release of these inflammatory mediators is increased cellular death of normal cells and even changes within the DNA of individual cells, which can result in the formation of cancer.  Cancer is essentially a syndrome in which a cell's DNA is altered, allowing different actions from that cell, different protein release and an altered and sometimes uncontrolled state of replication.  When this abnormality becomes unchecked, the disease can become enhanced and lead to a malignancy.  More on cancer in another section.
     This brings us full circle to our immune system and the role that it plays in our body.  Our immune system is comprised of various white blood cells, each with their own function.  The neutrophils are responsible for killing bacteria and yeast and are the first white blood cells to respond to a cite of infection.  Eosinophils are a type of cell associated with delayed type hypersensitivities such as allergies and parasitic infections.  Monocytes are scavengers and circulate throughout the body looking for anything abnormal.  Monocytes along with other antigen presenting cells, essentially take pieces of whatever they have consumed and present it to other cells like the lymphocyte.  The lymphocytes then read this information and then go about their job, very similiar to a coonhound sniffing clothing and then seeking a subject.  Lymphocytes cannot engulf material like monocytes, but instead act in a different manner.  Lymphocytes are broken up into two distinct classes: B and T lymphocytes.  The B-lymphocytes originate from the bone marrow and produce antibodies.  The antibodies bind to foreign material or abnormal cells, essentially labeling them for destruction.  T-lymphocytes originate from the thymus gland and are subdivided into 3 more classifications:  T-helper, T-suppressor and Cytotoxic T-cells.  The Cytotoxic T-cells are primarily responsible for killing a variety of cells including tumor cells or infected or abnormal cells.  Other immune cells that are important are dendritic cells, which are also antigen presenting cells and Natural Killer Cells (NK) which are important for cellular destruction as well.
     It has been stated that in order for certain disease to develop, the immune system must not be functioning correctly, either functioning poorly (immunocompromise) or being overactive (immunostimulation).  Various factors come into play, including influence of diet and nutrients, environmental factors, medications consumed and even stress.  The immune system must work like a symphony with each cell doing their respective job in harmony with the others and when the inflammatory response has performed its duty, the fire must be extinguished in order to prevent further damage to normal cells.  Many factors such as those mentioned above have been noted to either compromise our immune response or in some cases heighten the response uncontrollably.  For instance, individuals that are under a high amount of stress are prone to immunocompromise.  Why is this?  In instances of stress, our bodies as well as those of animals, secrete cortisol into the bloodstream.  Cortisol is a steroid hormone and has a defined role, however in high amounts, cortisol can actually suppress the immune system, predisposing to illness.  Another example are organ transplant recipients.  These individuals are destined for a life of immunosuppressive medications to prevent organ rejection.  In the process of lowering the immune system, the door is then opened for illnesses and even cancer.  Allergy sufferers that are on chronic use of steroids, either by oral or inhalation route, are also predisposed to various illnesses due to immunosuppression.  These are all instances or examples of immunosuppression, but what about cases of a heightened immune system?  Allergy sufferers are a prime example.  For some reason, in some instances, the immune system is always on RED ALERT and responds to simple things like pollen with a full military release of cytokines and eicosanoids into the bloodstream, leading to the common symptoms such as sneezing and watery eyes.  Why the immune system responds this way is still unclear, but it is definately an abnormality or defect in antigen presentation.  The common "cure" is steroid or antihistamine use, which suppresses the immune system.  Good, yes, however as we have seen there are side effects to this chosen therapy.
     The impact of the immune system is far reaching and not just associated with the common cold.  As we will see, the impact can extend out and include a higher predilection towards diseases such as cancer.