Humoral immunity describe those aspects of the immune system that generally appear in the body's fluids (such as blood and lymphatic fluid). The "Humor" is from the ancient belief that the body had four "humours" which, when out of balance, caused disease. In modern use, it describes the effect of antibodies and the other cells of the immune system that moderate the effect of antibodies. It is a sub-field of immunology.

The field began with the study of serum and how it was effective in the early days of modern medicine in fighting infections and toxins. This observation led to the isolation of complement proteins.

Our current understanding is that an immune response is triggered when a B cell comes into contact with a recognized foreign invader. This triggers the proliferation of identical B cells which create the specific antibodies to fight the disease in question. This reaction is moderated by the complement system.

Humoral immunity at Wikipedia