Antigens and Immunogens by Dr Samuel Munjita-unza

Antigens and Immunogens Dr Samuel Munjita Introduction Whenever we refer to a pathogen, most often we may think the entire microbe is responsible for the inducing the immune system. However, these organisms possess numerous sites that are recognised by antibodies or receptors on the cells such as B and T cells. In this lecture, we shall discuss antigens, immunogens, epitopes and the factors that influence immunogenicity Definitions: antigens and immunogens Antigen is any agent that is recognised by specific components of immune responses such as lymphocytes or antibodies and is capable of binding specifically to these components. Remember: T cell receptors only recognise antigens complexed with MHC molecules. Antigenicity is the ability of antibody or lymphocytes to bind specifically to antigens. Definitions: antigens and immunogens Although all antigens are recognised by specific lymphocytes or by antibodies, only some antigens are capable of activating immune responses. Antigens that are able to induce humoral or cell mediated immune responses or both are known as immunogens. Immunogenicity is the ability to induce humoral and cellular immune responses. All immunogens are also antigens. But, not all antigens are immunogens (not every antigen is able to trigger an immune response). Types of Antigens Antigens can be classified on the basis of their origin or on the basis of immune responses On the basis of their Origin 1. Exogenous antigens These antigens enters the body or system and start circulating in the body fluids and trapped by the APCs (Antigen presenting cells such as macrophages, dendritic cells, and activated B lymphocytes.) The uptakes of these exogenous antigens by APCs are mainly mediated by the phagocytosis. Some antigens start out as exogenous antigens, and later become endogenous (for example, intracellular viruses e.g. HIV) Types of Antigens 2. Endogenous antigens These are body’s own cells or sub fragments or compounds or the antigenic products that are produced. Examples: Blood group antigens. 3. Autoantigens An autoantigen is usually a normal protein or complex of proteins (and sometimes DNA or RNA) that is recognised 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. Types of Antigens On the basis of immune response 1. Complete Antigen or Immunogen These antigens are able to generate an immune response by themselves. High molecular weight (more than 10,000 Da) May be proteins or polysaccharides Types of antigens 2. Incomplete antigens Incomplete antigens are incapable of inducing immune responses by themselves but can induce immune responses upon binding to larger molecules called carriers. Incomplete antigens are referred to as Haptens. Haptens are low molecular weight (Less than 10,000 Da) compounds that can combine with antibody but cannot initiate an immune response unless it is coupled to a larger carrier molecule. Types of Antigens Incomplete antigens continued Types of Antigens Incomplete antigens continued The carrier molecule is a non-antigenic component and helps in provoking the immune response. Example of carrier molecules: Serum Protein such as Albumin or Globulin. Examples of Haptens include nickel. Nickel is a substance of small molecular weight which is incapable of provoking an immune response in its own right. Nickel allergy, however, is a common cause of contact dermatitis. This results when nickel combines with protein in the patient’s skin. The nickel-protein complex is recognised as foreign and an immune response is mounted. Penicillins, capsular polysaccharide of pneumococcus, polysaccharide “C” of beta haemolytic streptococci, and cardiolipin antigens are other examples of Haptens. Factors affect the immunogenicity (ability to induce immune responses: Factors affect the immunogenicity (ability to induce immune responses: Foreignness Immune system usually differentiate self and non self (Remember positive and negative selection). Therefore, the more foreignness of substances toward host, thee more immunogenic. 2. Molecular Size Immunogens with molecular mass of 100,000 daltons (Da) are more immunogenic. Substances with a molecular mass less than 5000–10,000 Da are poor immunogens. Factors affect the immunogenicity (ability to induce immune responses: 3. Chemical composition and heterogeneity Proteins are more immunogenic compared to other macromolecules. Nucleic acids and lipids are not immunogenic. Antibodies still can …