Test Your Knowledge on Antigens and the Immune System Recognition

Introduction to Antigens and Immunogens

• The section of the immune system covered in the rest of the course is the adaptive immune system, which primarily deals with the humoral arm.

• The non-specific immune system will still be discussed as there are interactions between the two arms of the immune system.

• An immunogen is any substance that induces a specific immune response.

• An antigen is anything that is capable of reacting with the products of the specific immune response.

• All immunogens are antigens, but not all antigens are immunogenic.

• Haptens are non-immunogenic substances that can react with the products of the specific immune response and have antigenicity.

• Epitopes or antigenic determinants are discrete parts of an antigen that induce the immune response and react with the products of the response.

• Antibodies are the products of the plasma cells that react with an antigen and mediate the humoral arm of the specific immune response.

• Factors that influence whether something is immunogenic include the foreignness, size, chemical composition, physical form, and degradability of the substance.

• Species and individual variability, genetics, age, the method of administration, and the use of adjuvants also play a role in the immune response.

• The optimal range of antigen or immunogen concentration and the route of administration also affect the response.

• Using adjuvants can boost the immune response to an immunogen.Antigenic Determinants and Immune Response

• Adjuvants are used to enhance the immune response to immunogens, but they can cause severe local reactions.

• Alum is a commonly used adjuvant in vaccines, but there are not many approved for human use.

• Proteins and polysaccharides are good immunogens due to their chemical complexity and the ability to generate different epitopes.

• Nucleic acids are immunogenic but not particularly good immunogens, except in cases like systemic lupus erythematosus.

• Lipids are usually non-immunogenic, except for some phospholipids that can trigger cell-mediated immune responses.

• Antigens are divided into two categories: T-independent and T-dependent, based on their requirement for T cell help in B cell antibody production.

• T-independent antigens, such as polysaccharides, have many copies of one or a few antigenic determinants and can activate a large proportion of B cells.

• T-dependent antigens, such as proteins, have a lot of different epitopes but relatively few of each kind.

• Haptens, which are not immunogenic alone, can induce an immune response when covalently attached to an immunogenic carrier molecule.

• B cells and antibodies can recognize protein, polysaccharide, nucleic acid, and hapten determinants, and can bind to sequence or conformational epitopes.

• T cells only recognize protein antigens and sequence determinants, and require the antigen to be processed and presented in the context of major histocompatibility complex proteins.

• Immunodominant epitopes are the limited number of epitopes that elicit the bulk of the immune response to an antigen, and tend to be located on the outer surfaces of the antigen.Antigens, Superantigens, and Innate Immune System Recognition

• Antigens must be processed and presented by major histocompatibility complex (MHC) molecules for T cell recognition.

• T cells cannot recognize conformational determinants and require presentation by MHC molecules.

• Lipids can be recognized by T cells through CD1, an MHC-like molecule.

• T cell epitopes are larger than B cell epitopes, ranging from 5-15 residues.

• Only peptides that can bind to MHC molecules can be presented to T cells, resulting in responders and non-responders.

• Superantigens activate a large population of T cells, up to one in four, unlike conventional antigens.

• Superantigens include toxins produced by bacteria and can cause hyperimmune responses and diseases.

• Innate immune system recognizes broad molecular patterns called pathogen-associated molecular patterns (PAMPs).

• Pattern recognition receptors (PRRs) recognize PAMPs and activate macrophages for an inflammatory response.

• Complement, mannose binding protein, scavenger receptor, and Toll-like receptors (TLRs) are examples of PRRs.

• TLRs recognize specific PAMPs, such as lipopolysaccharide in gram-negative bacteria and double-stranded RNA in viruses.

• TLRs tailor the innate immune response to the invading pathogen.