- An antigen is any substance that can trigger an immune response and be specifically recognized by the immune system. These molecules play a central role in both adaptive and innate immunity, serving as the targets that our immune system identifies as either self or non-self.
- The structural characteristics of antigens are crucial for immune recognition. Antigens typically have specific regions called epitopes or antigenic determinants that are recognized by antibodies or T cell receptors. These epitopes can be linear, consisting of a continuous sequence of amino acids, or conformational, formed by the three-dimensional folding of the molecule. The specificity of immune responses depends largely on how well these epitopes are recognized by immune cell receptors.
- Antigens can be classified into several categories based on their origin and properties. Exogenous antigens come from outside the body, such as bacteria, viruses, pollen, or food proteins. Endogenous antigens are generated within cells, including viral proteins produced during infection or tumor antigens in cancer cells. Autoantigens are self-molecules that become targets of immune responses in autoimmune diseases. Allergens are environmental antigens that trigger allergic responses in susceptible individuals.
- The processing and presentation of antigens are critical steps in initiating immune responses. Professional antigen-presenting cells internalize and process antigens into smaller peptides, which are then displayed on their surface via MHC molecules. For T cell recognition, antigens must be processed and presented in this way, while B cells can recognize intact antigens directly through their surface immunoglobulins.
- The concept of immunogenicity refers to an antigen’s ability to induce an immune response. Factors affecting immunogenicity include the antigen’s size, chemical complexity, and how different it is from the host’s own molecules. Understanding these properties is crucial for vaccine development and treating autoimmune diseases. Additionally, some substances called haptens become antigenic only when combined with larger carrier molecules.
