One of my favorite courses during my undergraduate studies at University of California, Santa Cruz in the late 1970s was immunology, the study of our protection from foreign macromolecules or invading organisms, and our responses to them. In a very basic way, the immune system has the job of patrolling our bodies, recognizing anything that is "not self" and defending the body against it.
We often think about white blood cells doing this job, but other tissues and cells are involved including the blood, lymphatic system thymus, spleen, skin, and mucous membranes.
So many characteristics of our unique immune system are based on our genetic inheritance - certain strengths and vulnerabilities are passed down generation to generation. There is also tremendous influence on the functioning of our immune system from current exposures, substances that can both enhance and impair the functioning of our immune system.
We know that people who are well nourished tend to have a stronger immune system that can protect them and keep them healthier. And those whose diet leaves them missing or depleted of certain ingredients may be less healthy and more easily infected with bacteria and viruses traveling in the community circuit.
Stress can tax our immune system, while exercise and adequate sleep supports it.
A very interesting and necessary characteristic of our immune system is that it has the capacity to develop memory of a sort. This is the whole basis for vaccination against certain diseases. When the body is presented with something it considers foreign or "not self", it will respond with an attempt to destroy the perceived invader by a cascade of chemical reactions. This often may include antibodies being formed against some particular part of the foreign protein (invader). Once the body has made antibodies against the foreign material - those antibodies may persist in very low titer for a time and the memory of how to make more remains in the tissues that made the antibodies initially. Then when you do get that particular toxin or antigen exposure in the future - the body, having memory of this foreign threat, will be able to move into action more quickly with a host of substances including antibodies, which are manufactured more quickly.
This is all well and good ... identifying and wiping out the bad guys, foreign invaders as they might be known. But this protective system can generate an inappropriate immune response against our own tissues. When it does, we suffer the consequences of an autoimmune disease process. This may occur in a variety of different tissues end organ systems.
It seems there is an inherited predisposition to develop certain autoimmune diseases. Occasionally a bacteria or virus will trigger antibodies that cross-react with our own tissue; mistaking, for example, heart tissue for streptococcal bacteria and ending up with a problem like rheumatic heart disease. The mechanism is not always the same. Sometimes the symptoms are localized to a given tissue type, like for example polymyalgia rheumatica which tends to affect large muscle groups causing weakness and pain. Other times, multiple organs are involved, as occurs with systemic lupus erythematosus (which affects skin joints, kidneys, heart, brain, blood cells etc.).
Some people have more than one type of autoimmune disease. This suggests that there is a more central process by which all this occurs. Recent literature has found an interesting association between celiac disease and autoimmune or Hashimoto's hypothyroid disease. Celiac disease is an inherited disease causing a sensitivity or intolerance to gluten, which is found in wheat, barley, rye, and triticale. When these substances containing gluten are ingested, a toxic reaction occurs in the small intestine affecting the delicate villi of the small intestine mucosa where nutrients are absorbed. For many years the condition was diagnosed with a small bowel biopsy that showed injury and flattening of the small bowel mucus lining. Now we can screen for celiac disease with a comprehensive celiac disease panel blood test. Symptoms include abdominal cramping, intestinal gas, chronic diarrhea or constipation, anemia, weight loss, disorders of calcium requiring tissues such as bone and dental enamel, aphthous ulcers and a condition known as dermatitis herpetiformis which shows up as blistering intensely itchy skin. Other patients may have significant intestinal damage without many symptoms.
It is known that people with celiac disease have more porous intestinal lining that allows for some proteins ("foreign") to pass through directly into the blood stream where they are perceived as invaders-- and antibodies are made against them. It is postulated that certain of these antibodies that are formed may cross-react with the thyroid gland, then attack the thyroid and cause a low thyroid state. Certain celiac patients who avoid gluten entirely in their diet have then seen improvement in their low thyroid condition.
Beyond defending against infectious diseases, and generating autoimmune diseases, the immune system also has the job of monitoring for developing cancer cells and destroying them ... but that's for another column...