In the previous article, published last week, we examined the basic functions of the immune system. We discussed the two types of immune systems and how they work, as well as digging into some of the nuts and bolts of each. In this article, we will examine how the various parts of the immune system work in greater detail.
We will examine the Innate Immune System first. Most of the Innate Immune System is composed of the skin but there are some more in-depth features as well. The first of these is inflammation. When the body detects an infection, it immediately rushes as much oxygen-rich blood to the infected area as possible. This is what turns an infected area red. The cells that are damaged by the infection produce two special chemicals, eicosanoids and cytokines. Eicosanoids are specialized chemical compounds made as needed by the body. They specialize in causing fever, pain, and coagulation of the bloodstream among other things. Those do not sound like positives, but they are a great help to the body. Fever raises the body temperature. Many microbes prefer a moderate temperature range to reproduce. A fever makes it harder for them to multiply. Pain alerts the brain that something is wrong. Blood coagulation prevents the body from losing too much blood to a simple paper cut. Coagulation is complex enough to warrant its own article. Cytokines are small proteins that regulate cell function. They do this by binding to specialized receptors on the surface of the cell. When they bind to a cell, it gives the cell a signal to change the function or perform a function not normally performed.
Inflammation is not the only function of Innate Immunity. Leukocytes are an important part of innate immunity as well. Leukocytes are white blood cells which perform a specialized cell function called phagocytosis. There are several types of leukocytes, but that is subject for another article. Phagocytosis is essentially the leukocyte engulfing a pathogen and breaking it down until nothing is left. This can also be performed on cellular waste and broken bits of cells. Some leukocytes travel freely through the bloodstream, seeking out their prey. Others are stationed in certain areas of the body. In the instance of an infection, a type of leukocyte is often the first to arrive.
A further instance of Innate immunity is the natural killer cells, which we passed over briefly in the first part of this article. A type of lymphocyte, natural killer cells are the reinforcements. If an infection is not immediately mopped up by the leukocytes, the natural killer cells come in to help deal with the pathogen, generally arriving approximately three days after the infection is first detected. The natural killer cells also target tumors, including cancerous ones. Instead of swallowing and digesting pathogens, natural killer cells do not attempt to deal with the pathogen itself. Instead, they tell the infected body cell to undergo a reaction called apoptosis. Apoptosis is the deliberately programmed death of a cell. This causes the pathogens within to either be killed with the cell or else be released into the bloodstream and the waiting jaws of the leukocytes. Of course, this has to be a carefully regulated process. If it was not, the body could end up killing off cells indiscriminately, weakening itself to the point of death.
Adaptive Immunity is even more complex than Innate Immunity. The human body produces what are called T cells. These come in three types. Killer T cells work by recognizing a specific antigen found on a particular pathogen. Once the body recognizes an infection, it sends a sample to the Killer T cells. Once the Killer T cells know what to look for, they activate and begin searching through the bloodstream for a cell bearing that antigen. Once they find it, they induce apoptosis in that cell, killing it. Helper T cells begin to work before that. They are the cells which assist the Killer T cells in finding their targets and in assuring that the B cells which we will discuss momentarily, mature correctly. They essentially find a potential pathogen and report it back to the Killer T cells, which come in to deal with it. The third variant, Gamma Delta T cells, are a relatively unknown quantity compared to the other two. They are believed to respond to lipid, or fat based antigens. They seem to cross over between the innate and adaptive immune systems, having some memory features of adaptive, yet performing phagocytosis like innate immunity.
Adaptive Immunity also contains the B cells. B cells are also lymphocytes, and they mature in the bone marrow of mammals. B cells work in three different manners. One is dependent on T cells for activation. The T cells are activated by receiving a particular antigen from a memory B cell (discussed below). They, in turn, activate the new B cells using cytokines. Once activated, the B cells produce special cells for short and long-term immune defense, along with more memory B cells for use against that pathogen in the future. In a second process, T cells are not necessary. Instead, the B cells notice antigens found on long protein chains or DNA from a virus and immediately activate, undergoing the same process as they would with T cell activation, producing the same short and long-term defenses as well as the memory B cells. Memory B cells are activated in both a T cell-dependent and independent fashion, depending on the type of antigen involved. Memory B cells enable the body to fight off a pathogen it has previously defeated, which explains why some diseases, such as chicken pox, are a once in a lifetime experience. These Memory B cells are built up over a lifetime so that the older a person gets, the more memory B cells they have, and thus the more diseases the body will recognize and defeat automatically. Babies are born with partial immunity from their mother. This passive immunity can last for as long as several months, enabling the infant to adjust to life outside the womb and helping it grow safely during those critical first few weeks.
As you can see, Immunity is a complex subject. What is above barely touches the amount of information available on immunity and does not come close to the complexity of the real thing. Bear that in mind in part three as we discuss the origin of immunity.