What is Humoral Immunity?

Humoral immunity is a means by which the body protects itself from infection by producing antibodies that target foreign material in the bloodstream that is seen as potentially dangerous, marking it for destruction. It is part of the adaptive immune system, which is activated in response to a specific threat, as opposed to the innate immune system, which is continually active but less effective. The other part of the adaptive system is cellular, or cell-mediated, immunity, in which cells release toxins to kill invaders or attack them directly, without the involvement of antibodies. Together, humoral and cellular immunity are designed to defend the body against a wide variety of threats that could compromise it.

This form of immunity starts in specialized white blood cells known as B-cells, which are produced by the bone marrow. They recognize antigens, which are certain molecules — such as some proteins — on the surface of a virus or bacterium. There are different types of B-cell, each designed to respond to a particular antigen. When one is encountered, the B-cell will multiply, producing huge numbers of individuals that release antibodies designed to attach to the antigen on the invading organism; they essentially turn into little antibody factories in the blood, floating around to target as many of the invaders as possible. Once marked by these antibodies, the invaders will be destroyed by other immune cells.

When the invader has been removed, many of the B-cells produced to counter this specific threat will die, but some will remain, settling in the bone marrow and acting as a kind of “memory” of this attack. People are born with a set of innate immune responses that are designed to recognize broad types of cells and organisms that could pose a threat to the body, but humoral immunity is acquired by being exposed to viruses, bacteria, and other substances that can cause harm. As time goes by, the body builds up more “memories” of previous assaults by harmful microorganisms.

The humoral immune response can produce a lasting immunity to many infectious agents. When the body comes under attack from an agent — such as a virus — that it has not encountered before, it has to start from scratch and typically takes several days to mount an effective immune response. During this time, the virus can multiply unchecked, causing an infection that may produce unpleasant, and possibly dangerous, symptoms. It is only when the body has produced large numbers of suitable antibodies that it can fight off the infection. If, however, it encounters this virus again, it will usually be much better prepared, thanks to the retention of B-cells produced in response to the previous attack, and it will be able to get to work on eliminating the invader immediately.

This immune "memory" is also how vaccination and immunizations work. People can be injected with dead or inactivated forms of a dangerous virus or bacterium that will stimulate a humoral immune response without posing any threat to the body. If, at some time in the future, this person is exposed to the real agent, there should be an immediate immune response that will eliminate it before it can do any serious damage.

Vaccination is more effective for some types of infection than for others. A worldwide vaccination program for the smallpox virus managed to bring about its complete extinction in the wild, as it was unable to find a human host who was not immune. Unfortunately, some viruses mutate rapidly, causing changes to the compounds on their surfaces that the humoral immune system uses to recognize them. This is why new influenza vaccines have to be continually developed. People vaccinated against this rapidly mutating virus may not be immune to a new strain that emerges the following year because the chemicals on its surface have changed and will not be recognized as antigens by the body’s B-cells.

When people develop problems with their humoral immunity, they are more susceptible to developing infection and disease. Conditions like HIV attack the immune system directly to make it less functional. Immunity can also be compromised by the use of certain medications, such as chemotherapy for the treatment of cancer and the drugs used to prepare people for organ transplant. In individuals who have compromised immune systems, aggressive and prompt treatment of any infection is critical to prevent the body from being overwhelmed by something that it cannot fight.

Another problem that can occur with the immune system is autoimmune disease. Normally, the system is able to distinguish chemically between substances that are part of the body and those that are not, and it will only respond to “foreign” substances. Sometimes, however, the system can mount an immune response to something that is a normal cell component in the body, treating it in the same way as an invading organism. This results in damage to tissues and is responsible for a number of serious diseases like rheumatoid arthritis, lupus, and celiac disease.

The term “humoral immunity” comes from the fact that this type of immunity is mediated by cells that float in the blood and lymph, or “humors” of the body. When researchers first began to explore the concept in the 1800s, many of them believed in medical theories dating back to ancient times, which included the idea that the balance of the body was maintained with substances that flowed through the body and caused various effects. While the humors theory has since been debunked, it lingers on in medical terminology.