By Barry C. Fox, M.D., University of Wisconsin
Besides innate immunity, there is a companion—the adaptive immune system. One difference between innate and adaptive immunity is that innate responses are nonspecific. Another difference is that innate immunity has no memory of previous actions against the pathogen. The two systems, though, work hand-in-hand.
The Concept of the Adaptive Immune System
The adaptive immune system is composed of highly specialized cells that adapt to and learn from prior invaders. This is a system that remembers, for example, that someone had measles as a child, and will protect the body for a lifetime against measles.
The adaptive system has two major branches, the cell-mediated system, and the antibody-mediated humoral system. Note that humor is an old word that describes body fluids—like lymph and blood.
Lymphocytes, one of the five types of white blood cells, carry out the immune responses in both branches of the adaptive system. Lymphocytes are divided into B-cells and T-cells.
Fetal primitive stem cells that continue to mature in the bone marrow become B-cells. Others complete their growth in the thymus and become T-cells. Importantly, each B- and T-cell is specific only for one antigen, hence, they can only bind to one particular molecular structure.
Note that antigen is short for antibody generator, and represents any foreign substance. Macrophages are another type of white blood cell—with macro meaning big, and phages meaning eater. They are present in essentially all tissues of the body and are critical in both innate and adaptive immunity.
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The Humoral vs. Cellular Immunity
B-cells work in the humoral immune system to mount a very specific antigen response. B-cells can also inactivate viruses by neutralizing them before they can enter host cells. Some B-cells become memory cells, allowing a quicker, more specific immune response the next time the body encounters the same infection.
T-cells, on the other hand, are part of a cell-mediated immune response and can be divided further into T-helper cells and cytotoxic T-cells. In the case of HIV or human immunodeficiency virus, the virus replicates rapidly and destroys the T-helper cells, resulting in more infected than healthy cells.
Cytotoxic T-cells attempt to destroy the HIV virus, but over time, the body’s ability to fight off infection is severely depleted—making people with HIV highly susceptible to infections. T-cells also activate B-cells, and they can destroy microbes that are inside cells. Some T-cells also transform into memory cells so that they will recognize an invader if it ever attacks again.
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Antibodies: Types and How They Function
Antibodies are the other major component of the humoral system. Antibodies are produced by B lymphocytes that have matured into plasma cells. They can perform several functions, including neutralizing bacterial toxins, binding to viruses to prevent entry into cells, and opsonization in which a coating of antibodies increases the effectiveness of neutrophils trying to engulf bacteria.
Antibodies are analogous to the pincers of insects and are generally Y-shaped. There are four major categories of antibodies—immunoglobulins M, A, E, and G.
IgM is a very large protein that clears foreign substances from the bloodstream. IgM antibodies are part of the body’s general first line of defense against invaders.
IgA is found in the secretions of the saliva and mucous. An IgA provides a mucosal first line of defense.
IgE is involved in immediate allergic responses, and sometimes, parasitic infections. IgE combines with a special cell known as a mast cell to release a chemical called histamine, which leads to swelling.
Finally, the most important immunoglobulin is IgG. IgG antibodies are formed in response to specific invaders, and they usually last a lifetime. They do take 7–14 days to be produced.
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The Use of IgG Antibodies to Cure Disease
IgG antibodies are produced in response to vaccination, but the two-week delay explains why it cannot be used as an immediate treatment strategy for infection. However, during the Ebola outbreak, another use of antibodies was tested—after the treatment and cure of the first American who had Ebola, doctors used the antibodies from his blood as a way to treat other patients with the Ebola virus.
The antibodies were transfused into newly infected patients in a process called passive immunization—providing immediate antibodies instead of waiting two weeks. A similar process is utilized after some tetanus exposures.
For those individuals who have unusually severe allergies, immunotherapy is also possible. This treatment involves the intentional introduction of foreign allergic antigens—under medical supervision—to decrease IgE and increase IgG and IgA production. The net result is an improvement in allergy symptoms.
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How the Immune System Functions in an Adaptive Fashion
When a child gets the live measles vaccine, what happens after vaccination? The vaccine with measles antigens stimulates B-cells to mature into either an antibody-secreting plasma cell (producing IgM and IgG antibodies) or memory B-cells.
This is also known as the primary immune response. The antibody levels will gradually decline once the threat is over, but the memory B-cells remain dormant and present in the lymphatic system.
If the individual contracts the measles virus again as an adult, similar to waking a bear out of hibernation, the cells will rapidly—within hours—start producing large amounts of measles-specific IgG antibodies. Antibodies will attach themselves to the virus before they can attack healthy body cells. The memory immune lymphocytes would also recognize a pathogenic germ and assist in its destruction.
Common Questions about the Adaptive Immune System—Cellular and Humoral Immunity
The adaptive immune system consists of specialized cells that adapt to and learn from previous invaders. For instance, this system remembers that someone had measles when they were a child, and when/if the same virus attacks their body again, this system will protect them.
Antibodies are a major component of the humoral immune system. The four major categories of antibodies are IgM, IgA, IgE, and IgG, with each one having a different function. ‘Ig’ is short for ‘immunoglobulins’.
IgE is a significant antibody in the adaptive immune system. IgE is involved in immediate allergic responses, and sometimes, parasitic infections. IgE combines with a special cell known as a mast cell to release a chemical called histamine, which leads to swelling.
