The humoral branch of the immune system defends the body by producing antibodies, substances that interact with microbes to kill them. The word "humoral" refers to antibodies. Immunoglobulin is another term you may hear used for antibodies.
Humoral immunity is most effective against bacteria and viruses that live outside of cells (extracellular microbes). The immune cells that produce antibodies are a special lymphocytes called activated B cells or plasma cells.
Several steps are required for the production of antibodies.
1. A white blood cell called a macrophage ingests (eats) an invading microbe. The microbe is digested by the macrophage (see Figure 1). Some of the microbe's digested proteins (antigens) are displayed by the macrophage on its surface to alert other cells of the immune system that an invader is present.
Figure 1: Macrophage Digesting Microbe and Displaying Antigen
2. Lymphocytes called B cells also process and display the invader's proteins on their surfaces (see Figure 2).
Figure 2: B Cell Digesting Microbe and Displaying Antigen
3. When an immune cell called a T helper cell sees the same protein on the surface of a B cell and a macrophage, it sandwiches itself between the two other immune cells (see Figure 3). The formation of this bridge complex stimulates the B cell to begin dividing, making more copies of itself. The resulting group of activated B cells produces antibodies against the invading microbe's displayed proteins (antigens).
Figure 3: T Helper Cell Activates B Cell Causing B Cell Expansion and Antibody Production
The antibodies produced against an invader attach to antigens on its surface. The presence of antibodies on the surface of the invader serves as a "red flag" to the rest of the immune system and marks the invader for destruction. The killing takes place in one of two ways. The antibodies may cause leaks in the outer coat of the microbe; the leaky invader cannot recover and dies. More commonly, antibodies on the surface of the invader alert the killer cells of the immune system to ingest (eat) and destroy the invader.
Figure 4: Antibody Tags Microbe Marking It For Immune System Destruction
Antigen-antibody interactions are very specific. Antibodies produced in response to a specific antigen normally reactonly with that antigen. Antigen-antibody interactions are often likened to a lock and key. A given lock can only be "activated" by a matching key. Similarly, and a given antibody only reacts with its matching antigen.