An antigen is any substance the immune system identifies as foreign and potentially harmful to the body, prompting an immune response. Antigens have two functional properties: immunogenicity and reactivity. Immunogenicity is the ability of an antigen to stimulate a specific immune response. At the same time, reactivity describes the antigen's ability to react with the cells and antibodies produced in response to it.

Complete Antigens

Complete antigens possess both immunogenicity and reactivity. Every complete antigen has specific regions known as antigenic determinants or epitopes. These epitopes are the specific molecular regions on antigens that the immune system recognizes and to which antibodies bind. A single antigen may have multiple epitopes, each capable of inducing a distinct immune response.

The chemical nature of antigens is diverse, encompassing proteins, polysaccharides, lipids, and nucleic acids. Their chemical composition defines their immunogenic properties and determines how the immune system recognizes and responds to them. Protein antigens are often the most potent in triggering an immune response due to their complexity and variability. They can have numerous and varied epitopes on their surface, each capable of binding to a specific antibody or immune cell receptor. Conversely, polysaccharide antigens, such as those found on the surface of many bacteria, are generally less effective at eliciting an immune response. However, they are crucial targets of the immune system, especially in bacterial infections. Lipid and nucleic acid antigens are less common and usually only immunogenic when combined with proteins or polysaccharides, forming lipoproteins, glycolipids, or nucleoprotein complexes.

Incomplete Antigens or Haptens

Unlike complete antigens, incomplete antigens, also known as haptens, are small molecules that lack immunogenicity. However, when they attach to larger protein molecules, they become immunogenic and can trigger an immune response. This phenomenon explains why certain small molecules can cause allergic reactions when they bind to proteins in the body, forming a complex that is recognized as foreign by the immune system. Common examples of haptens include certain medications such as penicillin, dyes, and components of animal venom.

Major Histocompatibility Complex Molecules

The body distinguishes self from non-self using self-antigens, known as the major histocompatibility complex (MHC). There are two classes of MHC molecules — MHC-I and MHC-II. All nucleated cells of the body carry MHC-I glycoproteins, which present endogenous antigens — peptides from within the cell — to cytotoxic T cells. This presentation ensures that cytotoxic T cells can target and destroy infected or abnormal cells. MHC-II glycoproteins are found on antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells. MHC-II molecules present exogenous antigens—peptides from outside the cell—to helper T cells to activate them. Once activated, helper T cells proliferate and secrete cytokines that coordinate and amplify the immune response and activate other immune cells like B cells and cytotoxic T cells.