Immunology The innate and adaptive immune systems
The immune system is the collection of cells, tissues and molecules that protects the body from numerous pathogenic microbes and toxins in our environment. This defense against microbes has been divided into two general types of reactions: 1- reactions of innate immunity 2- reactions of adaptive immunity. Thus, innate and adaptive immunity can be thought of as two equally important aspects of the immune system. Each aspect differs with respect to how quickly it responds and for how long it responds to pathogens, its central effector cell types and its specificity for different classes of microbes.
Innate immunity refers to nonspecific defense mechanisms that come into play immediately or within hours of an antigen's appearance in the body. These mechanisms include physical barriers such as skin, chemicals in the blood, and immune system cells that attack foreign cells in the body. The innate immune response is activated by chemical properties of the antigen.
The innate immune system consists of cells and proteins that are always present and ready to mobilize and fight microbes at the site of infection. The main components of the innate immune system are: 1) physical epithelial barriers 2) phagocytic leukocytes 3) dendritic cells 4) a special type of lymphocyte called a natural killer (NK) cell 5) circulating plasma proteins.
Adaptive immunity Adaptive immunity refers to an antigenspecific immune response. The adaptive immune response is more complex than the innate. The antigen first must be processed and recognized. Once an antigen has been recognized, the adaptive immune system creates an army of immune cells specifically designed to attack that antigen. Adaptive immunity also includes a "memory" that makes future responses against a specific antigen more efficient.
The adaptive immune system is act against pathogens that are able to evade or overcome innate immune defenses. Components of the adaptive immune system are normally silent; however, when activated, these components adapt to the presence of infectious agents by activating, proliferating, and creating potent mechanisms for neutralizing or eliminating the microbes.
There are two types of adaptive immune responses 1-humoral immunity, mediated by antibodies produced by B lymphocytes, 2- cell-mediated immunity, mediated by T lymphocytes
Antigenic characteristics An antigen is any substance that causes an immune system to produce antibodies against it Each antibody is specifically selected after binding to a certain antigen because of random somatic diversification in the antibody complementarity determining regions
In summary an antigen is a molecule that binds to Ag-specific receptors, but cannot induce an immune response in the body by itself. The term originally described a structural molecule that binds specifically to an antibody. It expanded to refer to any molecule or a linear molecular fragment that can be recognized by highly variable antigen receptors (B-cell receptor or T-cell receptor) of the adaptive immune system.
Other derivatives 1- Auto antigens, for example, are a person s own self antigens. Examples: Thyroglobulin, DNA, Corneal tissue, etc. 2- Allo antigens are antigens found in different members of the same species (the red blood cell antigens A and B are examples). 3- Heterophile antigens are identical antigens found in the cells of different species. Examples:, Cross-reacting microbial antigens, 4- Adjuvant are substances that are nonimmunogenic alone but enhance the immunogenicity of any added immunogen.
Types of antigens 1- On the basis of order of their 1- Exogenous antigens class (Origin) Exogenous antigens are antigens that have entered the body from the outside, for example by inhalation, ingestion or injection. The immune system's response to exogenous antigens is often sub clinical. By endocytosis or phagocytosis
2-Endogenous antigens Endogenous antigens are generated within normal cells as a result of normal cell metabolism, or because of viral or intracellular bacterial infection. 3-Autoantigens An autoantigen is usually a normal protein or complex of proteins (and sometimes DNA or RNA) that is recognized by the immune system of patients suffering from a specific autoimmune disease These antigens should not be, under normal conditions, the target of the immune system, but, due mainly to genetic and environmental factors, the normal immunological tolerance for such an antigen has been lost in these patients. Examples: Nucleoproteins, Nucleic acids, etc.
2- On the basis of immune response 1. Complete Antigen or Immunogen Posses antigenic properties denovo, i.e. there are able to generate an immune response by themselves. May be proteins or polysaccharides
2. Incomplete Antigen or Hapten A- These are the foreign substance, usually nonprotein substances B- Unable to induce an immune response by itself, they require carrier molecule to act as a complete antigen. C- The carrier molecule is a non-antigenic component and helps in provoking the immune response. Example: Serum Protein such as Albumin or Globulin. D- Haptens can react specifically with its corresponding antibody. Examples: Capsular polysaccharide of pneumococcus,