Antigens are substances that react with naive B cells when free-floating or with T cells when processed and displayed on cell surfaces along with MHC molecules.
Both antigens and haptens (small molecules) can act as antigenic agents when faced with an active immune response in progress. But only antigens themselves can cause an immune response to begin; they are both antigenic and immunogenic. Anything that can start an immune response can react with an in-progess immune response, but some compounds cannot start an immune response (although we can make antibodies to them and an in-progress response will react with them). This is the difference between something that is immunogenic and antigenic.
What makes something immunogenic (capable of creating a full immune response)?
- heterogeneity (different amino acids, rather than synthetic monopolymers)
- foreigness (must be different from host's proteins)
- size (must be relatively large, e.g. 100 kDalton)
Because of the nature of antibody molecules, proteins and polysaccharides are the most common humoral antigens. Proteins are broken down and presented with MHC molecules to produce the cell-mediated immune response, and lipids/glycolipids are similarly presented (although they are presented on CD1 at the cell surface). Leprosy and tuberculosis are presented to the T cells in this CD1/lipid manner.
Immunogens are often given deliberately, as in the case of vaccines. Often they are complexed with adjuvants, compounds that enhance the vaccine. Adjuvants can include broken-up bacterial cell walls, oil (to slow the dispersion of the vaccine into the body), and water. The addition of bacterial cell wall components induces the innate immune system to actuate, enhancing the adaptive system due to a higher concentration of MHC-II receptors on the APCs, resulting in more activated B cells.
Remember that epitopes are the parts of an antigen that elicit an immune response. It makes sense that B cell-recruiting epitopes are more likely to reside on the surface of foreign invaders, and these epitopes are likely to be hydrophilic. T cell epitopes are more likely to be larger peptides, cleaved and presented with MHC-I and MHC--II on the surface of body cells and APCs. Thus, the outer hydrophilic parts of an invader elicit B cell responses, and the relatively hydrophobic inner parts elicit T cell responses. Also, denatured proteins are less likely to create a B cell response, but T cells are used to interacting, via their CD4 and CD8 receptors and TCRs, with broken-up peptides (antigenic peptides).
Haptens must join with carrier proteins in order to produce an immune response. Drug allergies are almost always caused by drugs that are, by themselves, too small to be immunogenic. Penicillin and other drugs will bind with nucleophilic sidechains on circulating body proteins, forming a hapten-carrier conjugate. This causes antibodies (IgE) to the drug to be produced, and an allergic reaction begins when IgE-receptors on mast cells are activated. Treatment with epinephrine will reverse the anaphylactic shock, diverting blood from the periphery and dilating the airways in a classical sympathetic response. For this reason, persons with life-threatening allergies will often carry epinephrine (adrenaline) injections with them in case of exposure to the hapten.