Adaptive (Specific) Defenses

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Chapter: Anatomy and Physiology for Health Professionals: Lymphatic System and Immunity

Immunity, or adaptive specific defenses, also known as the third line of defense, is defined as resistance to specific pathogens or their toxins and metabolic byproducts.


Adaptive (Specific) Defenses

Immunity, or adaptive specific defenses, also known as the third line of defense, is defined as resistance to specific pathogens or their toxins and metabolic byproducts. Adaptive immune responses are carried out by lympho-cytes and macrophages that recognize and remember certain foreign molecules. As a fetus develops,­ cells learn to recognize proteins and large molecules as being “self.” The lymphatic system, as it develops, responds to “non-self” or foreign antigens and not, if the system is normal, the “self” antigens. Immunity is therefore “triggered” by initial exposures to antigens. After exposure, it can effec-tively protect the body. When immunity is disabled or fails to protect the body, serious diseases such as AIDS or cancer can develop.

A normal immune system protects the body against infectious agents and abnormal cells. The adap-tive immune system can greatly increase the inflam-matory response, and is responsible for the majority of complement activation.

Antigens

Antigens include proteins, polysaccharides, glyco-proteins, and glycolipids that are commonly found on cell surfaces. Antigens can mobilize adaptive defenses and cause an immune response. All adap-tive immune responses ultimately target antigens, which are usually large and complex molecules not normally present in the body. Antigens may be natu-ral or synthetic in nature and complete or incomplete. Complete antigens have both immunogenicity andreactivity. Immunogenicity is the ability to stimu-late certain lymphocytes to multiply. Reactivity is the ability to react with activated lymphocytes and antibodies that are released via immunogenic reac-tions. Proteins are the strongest types of antigens. Microorganisms and grains of pollen are immu-nogenic because their surfaces hold many different types of foreign macromolecules.

A small molecule, or incomplete antigen, which cannot stimulate an immune response by itself, is known as a hapten. It is found in certain drugs such as penicillin, in dust particles, animal dander, poison ivy, detergents, cosmetics, and in various household and industrial chemicals. Haptens usually combine with larger, more complex molecules to elicit an immune response.

Antigenic Determinants

The size and complexity of a molecule determines how it will be able to act as an antigen. The only parts of an antigen that are immunogenic are its antigenic determinants. Antibodies or lymphocyte recep-tors bind to antigenic determinants similarly to how enzymes bind to substrates. The majority of naturally occurring antigens have various antigenic determi-nants located on their surfaces. Some antigenic deter-minants are more potent than others, in relation to the immune response they cause. Different lympho-cytes react to different antigenic determinants. This means that a single antigen can mobilize several types of lymphocytes, causing the formation of many types of antibodies.

Larger proteins have hundreds of antigenic deter-minants, all with different chemical makeups. This explains their high reactivity and immunogenicity. However, there is little or no immunogenicity in large but simple molecules, such as plastics. These mol-ecules have many identical and regularly repeating units. Substances such as these plastics can be used to make artificial implants, since the body does not sense them as being “foreign,” and therefore, it does not reject the implants.

Self-Antigens: MHC Proteins

All body cells have many different protein mole-cules on their surfaces. In a normal immune system, self-antigens are not foreign or antigenic to thebody, but are extremely antigenic to other people. This concept is at the core of transfusion reactions and graft rejections. The MHC proteins are among the cell sur-face proteins that identify each cell as “self.” Genes of the MHC code for these proteins. There are millions of combinations of such genes that are possible. There-fore, it is not likely that any two individuals, except for identical twins, will have the same MHC proteins. Each of these proteins has a deep groove holding a peptide that is either a self-antigen or a foreign anti-gen. T lymphocytes only bind antigens presented to them on MHC proteins.

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