The Human Immune system
The human immune is extremely complex. It has evolved over hundreds of millions of years to respond to invasion by the pathogenic microbes that regularly attempt to infect our bodies, and invasion by the microbes that tried to infect our genetic ancestors. There are similarities between the immune system of humans and those of the most primitive of vertebrates, going back five hundred million years on the evolutionary ladder.
The immune system does not rely on one single mechanism to deter invaders, but instead uses many strategies, the most important of which are detailed below. The main division between the strategies is that between Innate immunity, which does not require previous exposure to the invading microbe, and Acquired immunity, whereby the immune system "remembers" how to deal with a microbe that it has dealt with before.
The Phagocytes
Phagocytes are the soldiers of the immune system, and provide innate immunity. They are responsible for swallowing, killing and digesting invading microbes. The process of swallowing microbes is known as phagocytosis. There are two main types of phagocyte
- Microphages. These cells are also known as Polymorphonuclear Leucocytes, PMNs and Polymorphs. These cells start life in the bone marrow. They are constantly circulating in the blood. They cannot replicate, and live for only a few days. The bone marrow contains large reserves of microphages.
- Macrophages. These cells start out life as monocytes, which originate in the stem cells in the bone marrow, but when they are first called into action, they turn into macrophages. Macrophages are not as numerous as microphages, and there are no large reserves of them, but they are longer lived than microphages. Macrophages are stationed at strategic locations throughout the body, usually in places that are not otherwise well defended. These areas include the alveoli of the lungs, the abdominal (peritoneal) and chest (pleural) cavities, under the top layer of the skin and the intestines. Macrophages are the front line of defense against microbial invasion in these areas.
As mentioned above, the process of swallowing of microbes by the phagocytes is known as phagocytosis. After the invading microbe has been ingested, the next task for the phagocyte is to kill the microbe. This is achieved in two main ways.
- Aerobically, i.e. using oxygen. The phagocytes produce oxygen based chemicals that are highly disruptive to the swallowed microbe. Oxygen is highly chemically reactive, and these oxygen based chemicals "tear" the microbe apart. This process is known as the oxidative burst, or the respiratory burst.
- Anaerobically, i.e. without using oxygen. One way to kill the microbe without oxygen is by using a chemical that deprives the microbe of iron, thus preventing it from metabolising. Another way is to increase the acidity of the internal environment of the phagocyte.
When these tasks are complete, the Macrophages have one further task to complete. They return to the lymph nodes, displaying the remnants of the destroyed invader on their surface. This has the effect of stimulating the cells of the Acquired immunity system into action.
The Complement System
The complement provides innate immunity. It is comprised of a collection of proteins that "recognise" corresponding proteins on the cell walls of invading microbes. When such invading microbes have been recognised, the following actions are taken
- The "alarm" is sounded. Chemicals, known as chemotaxins, that attract phagocytes are emitted. This process is known as chemotaxis. The phagocytes follow the trail of chemotaxins to arrive at the site of invasion.
- The invading bacteria are "marked" with chemicals that make them stand out. These chemicals are known as opsonins, from the Latin word opsonium, meaning "sauce". This "marking" greatly increases the chances of the invading bacteria being phagocytosed.
- Chemicals are released which promote the inflammatory response. More on the inflammatory response later.
Acquired Immunity
The acquired immunity system comprises B Cells and T Cells. Together, these cells which provide acquired immunity are known as Lymphocytes. The acquired immunity system further divides into two parts, humoral immunity and Cell Mediated Immunity (CMI).
B Cells provide "Humoral Immunity". Each B cell secretes a unique antibody, which acts against a particular antigen. An antigen is a chemical feature (a protein) which is unique to any given type of invading organism. When B cells meet an invading organism for which they have the antibody, they do one of two things.
- They may turn into antibody factories and start manufacturing as many copies of their antibody as they can.
- They may clone themselves, thus increasing the numbers of antibody factories, which results in an increased immune response to the target organism.
T Cells provide "Cell Mediated Immunity", often referred to as "CMI". T cells have several functions. They can be:-
- Helper T cells, which control other cells, such as B cells or Macrophages, directing them to carry out their task.
- Suppressor T cells, which dampen down the immune response when it is no longer needed.
- Cytotoxic T cells, which destroy host cells that have become infected with the invading organism.
Cytokines
Cytokines are the last element of the immune system which we shall discuss here. Cytokines (meaning "cell movers") are the messengers of the immune system. The above mentioned elements of the immune system (Complement, Phagocytes, Lymphocytes) do not work separately, but all work together in co-operative fashion. If they are to work effectively, they need a good system for communicating messages. This system is provided by the cytokines. Among the cytokines are the Interleukins, of which there are known to be at least twelve, Gamma-Interferon, Lymphotoxin, and Tumor Necrosis Factor.