Of all the systems of the body, it is the immune system that acts as the body's natural defense mechanism against disease. By monitoring the internal status of the body to determine what properly belongs and is healthy, the immune system sounds the alarm upon detecting the presence of foreign, non-compatible or even threatening substances. Upon sensing the invasion of some foreign substance, the immune system then determines if its presence is a threat–if it is damaging to healthy cells. The presence of such a substance such as a virus, would then signal an alert and demand a response. At this point the immune system creates antibodies–special cells designed to either neutralize or destroy the foreign substance within the body.
The immune system is one's sensitivity to foreign matter and can be understood on a broader spiritual plane to refer to a person's innate sense of self and non-self. A person is at home with himself–with what he perceives to be part of him. A person naturally recoils from what he senses to be non-self, some type of foreign invasion, be it on a biological level in the form of a disease or psychological in the sense of strange and undesired influences.
A problem of the immune system is therefore a problem related to the defense mechanism of the body. An extreme example of such a problem is in the case of an autoimmune disease, a disease that attacks the root of the body's defense mechanism, rendering it unable to distinguish between healthy and unhealthy elements; unhealthy elements appear to the body to be healthy. Furthermore, the immune system may then experience such internal confusion that it mistakenly perceives the person’s own healthy body as the threat–the ally, or one's own people, appears to be the enemy. In these cases, the immune system actually creates antibodies to fight against the healthy cells and organs of the body itself.
In the words of the prophet Isaiah:
Woe unto them that call evil good and good evil;
that put darkness for light and light for darkness;
that put bitter for sweet and sweet for bitter!
In all of the three psychological misinterpretations that characterize the nature of sickness, as depicted here by the prophet–evil vs. good, darkness vs. light, bitter vs. sweet–he who suffers from disease first sees the negative to be positive (evil to be good, etc.), and only thereafter, in consequence, sees the positive to be negative. And so with regard to the body: in an autoimmune disease, the body first misinterprets the invading, unhealthy matter to be healthy, and then continues to fight against its own healthy cells, as though they were foreign invaders.
In Kabbalah we are taught that the word "woe" (hoy) which begins the above words of the prophet refers to an existential state of "disappearance" of spiritual light and life force from the body. In particular, that life force that extended, through the means of the blood–the physiological system that corresponds to thesefirah of binah–to all the limbs of the body (in Kabbalah, from chesed to hod) returns and disappears from the body to its source in the unconscious realm of the mind. The mind is no longer able to affect the limbs, to permeate the body with the innate ability to understand what is good (for me) and what is bad (for me). As we shall see further, all disease relates to the well-being of the blood and the circulatory system. It is the function of the blood to bring "understanding"–binah–to all the cells of the body. "Woe" to the malfunction of the blood as reflected in the misunderstanding of the immune system.