All cells have many receptors on their cell surface. When antibodies or drugs bind to a receptor, a cell is triggered to perform a certain function or behave in a certain manner. Drugs can target disease-causing cells by binding to a receptor, but in some cases, disease-causing cells do not have unique receptors and therefore drugs also bind to healthy cells and cause "off-target" side effects.
Rituximab (Rituxan, Genentech), for example, is used to treat rheumatoid arthritis, non-Hodgkin's lymphoma and chronic lymphocytic leukemia by docking on CD20 receptors of aberrant cells that are causing the diseases. However, certain immune cells also have CD20 receptors and thus the drug can interfere with a person's ability to mount a fight against infection.