Difference between Antigens and Immunogens

Not all antigens are immunogens because the term “immunogen” refers to a specific subset of antigens that have the ability to trigger an immune response. Immunogenicity is a property of certain antigens that makes them effective at inducing an immune response, while other antigens lack this property for various reasons. Here are some key factors that explain why not all antigens are immunogens:

1. Size, weight and Complexity: Immunogens are often larger and more complex molecules, which have 8000-10000 Dalton molecular weight compared to non-immunogenic antigens. Large, complex molecules tend to be better at activating immune cells and initiating an immune response. Small, simple molecules may not be sufficient to stimulate the immune system effectively.
   Ex: The digested food material contains small molecules such as amino acids, monosaccharides etc do not initiate an immune response in our body.
2. Structure and Conformation: The three-dimensional structure and conformation of an antigen play a critical role in its immunogenicity. Some antigens may lack the necessary structural features to interact with immune receptors and initiate a response.
3. Stability: Immunogens are typically stable and resistant to degradation. Antigens that are easily broken down or denatured may not persist long enough to be recognized by immune cells and generate a response. Certain antigens are denatured by blood PH, the temperature of the body and lysosomal activity etc could not be recognised by our immune cells on the entry of antigens.
4. Exposure to Immune Cells: For an antigen to be immunogenic, it must come into contact with immune cells such as T cells, B cells, and antigen-presenting cells. Antigens that are sequestered within cells or tissues may not be readily accessible to immune cells.
5. Tolerance Mechanisms: The immune system has mechanisms to establish tolerance to self-antigens, preventing autoimmune reactions. Self-antigens are usually not immunogenic because tolerance mechanisms dampen immune responses against them.
6. Suppressive Factors: Some antigens may be present in environments where immune responses are actively suppressed, such as certain immune-privileged sites in the body. In such cases, immune responses to those antigens are inhibited. Ex: Immunosuppressive cells such as tumour-associated macrophages, neutrophils, regulatory T cells (Tregs), myeloid-derived suppressive cells, and tumour-associated, and tumour-associated dendritic cells are critical factors correlated with immune resistance.
7. Species-Specific Differences: What is immunogenic in one species may not be immunogenic in another. An antigen that induces a strong immune response in one species may have a weaker or no response in a different species. Covid-19 disease has not appeared in dogs.
8. Individual Variability: Immunogenicity can vary among individuals. What triggers a robust immune response in one person may not do the same in another due to genetic factors and prior exposure to similar antigens.
9. Context and Co-Stimulation: The presence of co-stimulatory signals and the overall immune context in which an antigen is encountered can influence its immunogenicity. Without the appropriate co-stimulation, an antigen may not induce a strong response.

Briefly, while all immunogens are antigens, not all antigens are immunogens due to variations in size, structure, stability, exposure to immune cells, and the regulatory mechanisms of the immune system. Immunogenicity is a complex property that depends on multiple factors, and it is a crucial consideration in vaccine design and immunology research.

Here are examples of antigens and immunogens:

Antigen Examples:

1. Influenza Virus Surface Proteins: The surface proteins of the influenza virus, such as hemagglutinin and neuraminidase, are antigens. They are recognized by the immune system when an individual is infected with the flu.
2. Pollen Grains: Pollen grains from plants can act as antigens, triggering allergic reactions in susceptible individuals. The immune system recognizes pollen as foreign and mounts an allergic response.
3. Bacterial Cell Wall Components: Components of bacterial cell walls, like lipopolysaccharides (LPS) in Gram-negative bacteria and peptidoglycans in both Gram-negative and Gram-positive bacteria, are antigens. They are recognized by the immune system during bacterial infections.
4. Self-Antigens: Some antigens are part of the body’s own tissues and are recognized as “self” by the immune system under normal circumstances. However, autoimmune diseases can occur when the immune system mistakenly targets self-antigens as if they were foreign.

Immunogen Examples:

1. Tetanus Toxin: The tetanus toxin is an immunogen. When introduced as part of the tetanus vaccine, it induces the production of antibodies and activates immune responses to protect against tetanus.
2. Inactivated Poliovirus: Inactivated poliovirus is used as an immunogen in the polio vaccine. It stimulates the immune system to produce antibodies and develop immunity to the polio virus.
3. Protein Subunit Vaccines: Many vaccines use protein subunits from pathogens as immunogens. For example, the human papillomavirus (HPV) vaccine contains virus-like particles (VLPs) that serve as immunogens to elicit an immune response against HPV.
4. Inactivated Influenza Vaccine: The inactivated influenza vaccine contains inactivated influenza virus particles, which act as immunogens. These particles stimulate the immune system to produce antibodies and provide protection against influenza.
5. Live Attenuated Vaccines: Live attenuated vaccines, such as the measles, mumps, and rubella (MMR) vaccine, contain weakened forms of pathogens that serve as immunogens. They induce an immune response while causing a mild or asymptomatic infection.
6. Antigenic Regions of Pathogens: Specific regions or epitopes on the surface proteins of pathogens, like HIV envelope glycoproteins or the spike protein of SARS-CoV-2, are immunogens. Vaccines target these regions to generate immunity against the respective pathogens.

Conclusion:

antigens are substances recognized by the immune system, while immunogens are a subset of antigens that can effectively stimulate an immune response, often leading to immunity against specific diseases. Vaccines are designed to contain immunogens that mimic the antigens found in pathogens, allowing the immune system to recognize and remember them without causing the disease.