Immunity is the final line of defense against disease. Immunity to disease can be defined as an individual’s power to resist or overcome the effects of a particular disease agent or its harmful products. In a broader sense, the immune system will recognize any foreign material and attempt to rid the body of it, as occurs in tissue transplantation from one individual to another. Immunity is a selective process; that is, immunity to one disease does not necessarily cause immunity to another. This selective characteristic is called specificity. There are two main categories of immunity:
* Inborn immunity is inherited along with other characteristics in a person’s genes.
* Acquired immunity develops after birth. Acquired immunity may be obtained by natural or artificial means; in addition, acquired immunity may be either active or passive.
     Figure 13-1 summarizes the different types of immunity. Refer to this diagram as we investigate each category in turn.
Types of immunity
Figure 13-1 Types of immunity.

Inborn Immunity

Both humans and animals have what is called a species immunity to many of each other’s diseases. Although certain diseases found in animals may be transmitted to humans, many infections, such as chicken cholera, hog cholera, distemper, and other animal diseases, do not affect human beings. However, the constitutional differences that make human beings immune to these disorders also make them susceptible to others that do not affect different species. Such infections as measles, scarlet fever and diphtheria do not appear to affect animals who come in contact with infected humans.
     Some members of a given group have a more highly developed individual immunity to specific diseases. For example, some people are prone to cold sores (fever blisters) caused by herpes virus, whereas others have never shown signs of this type of infection. Newspapers and magazines sometimes feature the advice of an elderly person who is asked to give his or her secret for living to a ripe old age. Some elderly people may say that they lived a carefully regulated life with the right amount of rest, exercise, and work, whereas others may boast of drinking alcohol, smoking, not exercising, and other kinds of unhealthy behavior. However, it is possible that the latter group resisted infection and maintained health despite their habits, rather than because of them, thanks to inherited resistance factors.
Acquired Immunity
Unlike inborn immunity, which is due to inherited factors, acquired immunity develops during a person’s lifetime as that person encounters various specific harmful agents. If the following description of the immune system seems complex, bear in mind that from infancy on, your immune system is able to protect you from millions of foreign substances, even synthetic substances not found in nature. All the while, the system is kept in check, so that it does not usually overreact to produce allergies or mistakenly attack and damage your own body tissues.
Antigens An antigen (Ag) is any foreign substance that enters the body and induces an immune response. (The word is formed from antibody + gen because an antigen stimulates production of antibody.) Most antigens are large protein molecules, but carbohydrates and some lipids may act as antigens. Antigens may be found on the surface of pathogenic organisms, on the surface of red blood cells and tissue cells, on pollens, in toxins, and in foods. The critical feature of any substance described as an antigen is that it stimulates the activity of certain lymphocytes classified as T or B cells.

T Cells Both T and B cells come from hematopoietic (blood-forming) stem cells in bone marrow, as do all blood cells. The T and B cells differ, however, in their development and their method of action. Some of the immature stem cells migrate to the thymus and become T cells, which constitute about 80% of the lymphocytes in the circulating blood. While in the thymus, these T lymphocytes multiply and become capable of combining with specific foreign antigens, at which time they are described as sensitized. These thymus-derived cells produce an immunity that is said to be cell-mediated immunity. There are several types of T cells, each with different functions. The different types of T cells and some of their functions are as follows:
* Cytotoxic T cells (Tc) destroy foreign cells directly.
* Helper T cells (Th) release substances known as interleukins (IL) that stimulate other lymphocytes and macrophages and thereby assist in the destruction of foreign cells. (These substances are so named because they act between white blood cells). There are several subtypes of these helper T cells, one of which is infected and destroyed by the AIDS virus (HIV). The HIV-targeted T cells have a special surface receptor (CD4) to which the virus attaches.
* Regulatory T cells (Treg) suppress the immune response in order to prevent overactivity. These T cells may inhibit or destroy active lymphocytes.
* Memory T cells remember an antigen and start a rapid response if that antigen is contacted again.
The T cell portion of the immune system is generally responsible for defense against cancer cells, certain viruses, and other pathogens that grow within cells (intracellular parasites), as well as for the rejection of tissue transplanted from another person.
The Role of Macrophages Macrophages are phagocytic white blood cells derived from monocytes (their name means “big eater”). They act as processing centers for foreign antigens. They ingest foreign proteins, such as disease organisms, and break them down within phagocytic vesicles (Fig. 13-2). They then insert fragments of the foreign antigen into their plasma membrane. The foreign antigens are displayed on the macrophage’s surface in combination with antigens that a T cell can recognize as belonging to the “self.” Self antigens are known as MHC (major histocompatibility complex) antigens because of their importance in cross-matching for tissue transplantation. They are also known as HLAs (human leukocyte antigens), because white blood cells are used in testing tissues for compatibility. Macrophages and other cells that present antigens to T cells are known as APCs (antigen-presenting cells). For a T cell to react with a foreign antigen, that antigen must be presented to the T cell along with the MHC proteins. A special receptor on the T cell must bind with both the MHC protein and the foreign antigen fragment (see Fig. 13-2). The activated Th then produces interleukins (ILs), which stimulate other leukocytes, such as B cells. There are many different types of interleukins, and they participate at different points in the immune response. They are produced by white cells and also by fibroblasts (cells in connective tissue that produce fibers) and by epithelial cells. Because ILs stimulate the cells active in immunity, they are used medically to boost the immune system.
Activation of a helper T cell by a macrophage
B Cells and Antibodies An antibody (Ab), also known as an immunoglobulin (Ig), is a substance produced in response to an antigen. Antibodies are manufactured by B cells (B lymphocytes), another type of lymphocyte active in the immune system. These cells must mature in the fetal liver or in lymphoid tissue before becoming active in the blood. B cells have surface receptors that bind with a specific type of antigen (Fig. 13-3). Exposure to the antigen stimulates the cells to multiply rapidly and produce large numbers (clones) of plasma cells. Plasma cells produce antibodies against the original antigen and release these antibodies into the blood, providing the form of immunity described as humoral immunity (the term humoral refers to body fluids). Humoral immunity generally protects against circulating antigens and bacteria that grow outside the cells (extracellular pathogens). All antibodies are contained in a portion of the blood plasma called the gamma globulin fraction. Some antibodies produced by B cells remain in the blood to give longterm immunity. In addition, some of the activated B cells do not become plasma cells but, like certain T cells, become memory cells. On repeated contact with an antigen, these cells are ready to produce antibodies immediately. Because of this “immunologic memory,” one is usually immune to a childhood disease after having it.
Activation of B cells
Figure 13-2 Activation of a helper T cell by a macrophage (antigen-presenting cell).
Figure 13-3 Activation of B cells. The B cell combines with a specific antigen. The cell divides to form plasma cells, which produce antibodies. Some of the cells develop into memory cells, which protect against reinfection.
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