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Antigen

 

INTRODUCTION
  • In immunology, an antigen (Ag) is any substance that can bind to a specific antibody or T-cell receptor triggering an immune response.This signifies that your immune system does not recognize the substance and is fighting it off by creating antibodies.
LEARNING OBJECTIVES
  • Understand what an Antigen is and its role in the immune system.
  • Identify Types of Antigens: Learn about the different types of antigens, including auto-antigens, hetero antigens, and alloantigens.
  • Antibody-Antigen Binding: Learn the mechanisms of antibody-antigen binding and specificity.

ANTIGENS AND THEIR CHARACTERISTICS

  1. An antigen is a substance that triggers an immune response, leading to the production of antibodies by lymphocytes.
  2. Recognition: Antigens are recognized by antigen receptors, including antibodies and T-cell receptors. They can encompass proteins, peptides, polysaccharides, lipids, or nucleic acids found on various entities like cancer cells, parasites, fungi, viruses, and bacteria.
  3. Antibody Specificity: Antibodies are specific to particular antigens; each antibody binds to one specific antigen. However, in some cases, antibodies may cross-react with multiple antigens, leading to the antigen-antibody reaction.
  4. Origin of Antigens: Antigens can either originate from within the body (self-antigens) or come from the external environment (non-self). The immune system identifies and targets external, non-self antigens. Autoimmune diseases occur when antibodies react with self-antigens, causing harm to the body's own cells.
  5. Vaccines: Vaccines contain antigens in an immunogenic form, intentionally administered to stimulate the adaptive immune system's memory function against pathogens. The seasonal influenza vaccine serves as a common example.
IMMUNOGEN
  1. Definition: An immunogen is a substance that independently triggers an immune response, leading to antibody production.
  2. Characteristics: Effective immunogens are usually large molecules with a molecular weight greater than ≈6000 and are chemically complex. An example is the surface antigen of the hepatitis B virus.
  3. Relation to Antigens: An immunogen is also an antigen, but not all antigens are immunogens.
HAPTEN
  • Definition: Haptens are compounds capable of binding to immunologic receptors but do not inherently provoke an immune response.
  • For instance, simple compounds like penicillin cannot induce an antibody response on their own.

TYPES OF ANTIGENS

  1. Exogenous Antigens: These originate from the external environment, entering the host. Examples include microorganisms, drugs, pollens, pollutants, and even food, causing diseases ranging from infections to immunological conditions like bronchial asthma.
  2. Endogenous Antigens: These are found within the host's own cells, either belonging to "self" or being of foreign origin. Normally, the immune system doesn't react to self-antigens, but certain circumstances can make them immunogenic, such as genetically mutated or cancerous cells.

ANTIGEN AND HOST RELATIONSHIP

  1. Autoantigens: These antigens belong to the host itself and typically do not trigger an immune response. However, in specific situations, they become immunogenic, leading to autoimmune diseases.
  2. Alloantigens: Alloantigens are derived from members of the same species as the host but not from the host itself. They are significant in tissue transplantation and blood transfusion, such as antigens present on donor and recipient RBCs.
  3. Heteroantigens: These antigens come from different species, such as plants, animals, or microorganisms. Heterophile antigens are a subtype of heteroantigens found in unrelated species, where their structures are similar enough that antibodies to one can cross-react with another. For example, human blood group A antigen and pneumococcal polysaccharide type XIV antigen are heterophile antigens.

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