Humoral Immunity refers to the production of antibody molecules in response to an antigen. These antibody molecules circulate in the plasma of the blood and enter tissue and organs via the inflammatory response. Humoral immunity is most effective microbes or their toxins located in the extracellular spaces of the body. Antibodies or immunoglobulins are specific glycoprotein configurations produced by B-lymphocytes and plasma cells in response to a specific antigen and capable of reacting with that antigen. In this section we will look at the structure of antibodies, the 5 classes or isotypes of human antibodies, generation of antibody diversity, clonal selection and clonal expansion, and memory (anamnestic) response.