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Immune Response
The immune system is a very complex interactive network of organs, tissues, cells, and biochemical mediators designed to counteract invasion of the body by nonself entities, such as bacteria, viruses, parasites, foreign tissue, and other endogenous threats including malignancies. The cellular and molecular components of the immune system recognize invading entities as nonself and mount an effector immune response, ultimately leading to the elimination of the foreign material. The ability of micro-organisms to cause disease depends upon their degree of pathogenicity and the integrity of the immune response.

An immune response is how your body recognizes and defends itself against bacteria, viruses, and substances that look to be foreign and harmful to the body. The immune system protects the body from potentially harmful substances by recognizing and responding to antigens. Antigens are molecules (usually proteins) on the surface of cells, viruses, fungi, or bacteria. Nonliving substances such as toxins, chemicals, drugs, and foreign particles (such as a splinter) can be antigens. The immune system recognizes and destroys substances that contain these antigens.

Now even your own body cells have proteins that are antigens. These include a group of antigens called human leukocyte antigens (HLAs). This is where the immune system learns to see these antigens as normal and doesn't usually react against them.

Innate Immunity

Innate, or nonspecific, immunity is a defense system that you are born with and should protect you against all antigens. The innate immune response provides a first-line, non-antigen-specific response to nonself material. Innate immunity involves barriers that keep harmful materials from entering your body including physical, chemical, and microbiological barriers.

Some examples of anatomical innate immunity include:

Cough reflex
Enzymes in tears and skin oils
Mucus - which traps bacteria and small particles
Skin
Stomach acid

Innate immunity also comes in a protein chemical form, called innate humoral immunity. Examples of this would include the body's complement system and substances called interferon and interleukin-1 (which causes fever). If an antigen gets past these barriers, it's attacked and destroyed by other parts of the immune system.

The white blood cells involved in innate immunity and each type having a different function are: Monocytes (which develop into macrophages), neutrophils, eosinophils, basophils, and natural killer cells. The complement system and cytokines also participate in innate immunity.

Acquired Immunity

Acquired immunity, also called adaptive immune response, is an immunity that develops with exposure to various antigens over time. Your immune system builds a defense that is specific to that antigen as well as many others. This type of response can be characterized by antigen-specific reactions through T lymphocytes or T cells (for cell-mediated immunity) and B lymphocytes or B cells (for producing antibodies).

Lymphocytes are the type of white blood cell responsible for acquired immunity. Typically, an acquired immune response begins when antibodies, produced by B cells, encounter antigen. Dendritic cells, cytokines, and the complement system (which enhances the effectiveness of antibodies) are also involved.

As a person's immune system encounters foreign substances (antigens), the components of acquired immunity learn the best way to attack each antigen and begin to develop a memory for that antigen. Acquired immunity is also called specific immunity because it tailors its attack to a specific antigen previously encountered. Its hallmarks are its ability to learn, adapt, and remember. Acquired immunity takes time to develop after initial exposure to a new antigen. However, because a memory is formed, subsequent responses to a previously encountered antigen are more effective and more rapid than those generated by innate immunity.

This type of immunity is usually slow to begin a response to a new antigen, but over several days of clonal expansion by T and B cells, the response quickens rapidly. This type of system has a memory, so any subsequent antigen exposure is met with a rapid response.

Passive Immunity

Passive immunity involves antibodies that are produced in a body other than your own. Infants have passive immunity because they are born with antibodies that are transferred through the placenta from the mother. These antibodies will typically disappear between 6 and 12 months of age.

Passive immunization involves injection of antiserum, which contains antibodies that are formed by another person or animal. It provides immediate protection against an antigen, but doesn't provide long-lasting protection. Gamma globulin given for hepatitis exposure and tetanus antitoxin are examples of passive immunization. This is why if you have cut yourself and can't remember when your last tetanus shot was, a doctor will give you another one while treating that cut.

Blood Cells

The immune system includes certain types of white blood cells. It also includes chemicals and proteins in the blood, such as antibodies, complement proteins, and interferon. Some of these directly attack foreign substances in the body, and others work together to help the immune system cells.

Lymphocytes are white blood cells, which includes B and T cells.

B cells produce antibodies that will attach to a specific antigen and make it easier for the immune cells detect it.

T cells attack antigens directly and help control of the immune response. They also release chemicals, known as interleukins, which control the entire immune response.

As lymphocytes develop, they normally learn to tell the difference between your own body tissues and substances that aren't normally found in your body. Once B and T cells are formed, a few of those cells will multiply and provide "memory" for the immune system. This allows the immune system to respond faster and more efficiently the next time you are exposed to the same antigen and in many cases will prevent you from getting sick. So when an individual who has had chickenpox or immunized against it, they are therefore immune from getting chickenpox again.

Inflammation

The inflammatory response or inflammation occurs when tissues are injured by bacteria, trauma, toxins, heat, or any other cause. The damaged tissue releases chemicals including histamine, bradykinin, and serotonin. These chemicals cause blood vessels to leak fluid into the tissues, causing swelling. This helps isolate the foreign substance from further contact with or spreading to other body tissues.

The chemicals also attract white blood cells called phagocytes that basically eat microorganisms and dead or damaged cells during a process called phagocytosis. The phagocytes eventually die, and then pus is formed from a collection of dead tissue, dead bacteria, and live and dead phagocytes.

Immune System Disorders and Allergies

Immune system or autoimmune disorders occur when the immune response is inappropriate, excessive, or lacking. Immunodeficiency disorders would also be an example of when the immune response is lacking. Allergies involve an immune response to a substance that most people's bodies perceive as harmless. Additional information can be found in the Immune Disorders section.

Immunization

Vaccination or immunization is a way to trigger an immune response. This can be done with small doses of an antigen, such as dead or weakened live viruses, that are given to activate immune system "memory" (activated B cells and sensitized T cells). This memory allows your body to react quickly and efficiently to future exposures.

Immune Tolerance

Immune tolerance is the tendency of T or B cells to ignore the body's own tissues. Maintaining tolerance is important because it prevents the immune system from attacking its fellow cells. It's still not fully understood how the immune system knows when to respond and when to ignore an antigen.

Tolerance occurs in at least two ways—central tolerance and peripheral tolerance. Central tolerance occurs during lymphocyte development when very early in each immune cell's life, it's exposed to many of the self molecules in the body. If it encounters these molecules before it has fully matured, the encounter activates an internal self-destruct pathway, and the immune cell dies. This process, called clonal deletion, helps ensure that "self-reactive" T and B cells, those that could develop the ability to destroy the body's own cells, don't mature and attack healthy tissues.

Because maturing lymphocytes don't encounter every molecule in the body, they must also learn to ignore mature cells and tissues. In peripheral tolerance, circulating lymphocytes might recognize a self molecule but can't respond because some of the chemical signals required to activate the T or B cell are absent. So-called clonal anergy, therefore, keeps potentially harmful lymphocytes switched off. Peripheral tolerance may also be imposed by a special class of regulatory T cells that inhibits helper or cytotoxic T cell activation by self antigens.