Chemically complex . Proteins are more complex, and make for more potent antigens, than carbohydrates. Lipids are not very antigenic. Small molecules are not visible to the immune system. Hapten refers to a substance that is foreign to the host, but that does not lead to antibody formation and is not recognized by the host immune system, usually due to being too small or too chemically simple.
Drugs are common examples of haptens.
If combined with another molecule, perhaps even a host cell molecule, haptens may be seen as antigen and attacked by the immune system. A common example of the latter is drug-induced hemolytic anemia, in which a drug binds to red blood cell membrane molecules, is then seen as antigen, and the cells are attacked and lysed, resulting in fever and shock. When a microbe invades the body, it percolates through the system until its antigens encounter matching receptors on a B-lymphocyte immune system cell in bone marrow, lymph nodes, or lymph nodules. The B cells then transform to plasma cells that manufacture massive amounts of antibody molecules. Antibody is secreted with the intention of percolating through the host system and attaching to and disabling pathogen or antigen.