Humoral Immune Responses

In the other adaptive immune response, humoral immunity, the immune system targets pathogens circulating in extracellular fluids including blood and lymph.

Invading pathogens such as bacteria are detected by specific white blood cells called B cells which recognize specific antigens on bacterial surfaces.

Once B cells are activated they proliferate and differentiate into plasma cells which secrete millions of antibodies that circulate throughout the body and activate several defense mechanisms.

In one method, antibodies that bind antigens on the surface of a pathogen can inactivate or neutralize it by interfering with its ability to infect the host cell.

They can also opsonize or tag pathogens for engulfment and destruction by phagocytes such as macrophages or neutrophils.

Finally, antibodies can activate the complement system, a complex of proteins that further enhances the opsonization and destruction of pathogens.

Even with the pathogens destroyed some activated B cells differentiate into Memory B cells instead of plasma cells. These Memory B cells continue to produce small amounts of antibody long after an infection has cleared. If the same pathogen reenters the body these circulating antibodies can target it for immediate destruction.