(See also Overview of the Immune System.)
Acquired (adaptive or specific) immunity is not present at birth. It is learned. The learning process starts when a person’s immune system encounters foreign invaders and recognizes nonself substances (antigens). Then, the components of acquired immunity learn the best way to attack each antigen and begin to develop a memory for that antigen. Acquired immunity is also called specific immunity because it tailors its attack to a specific antigen previously encountered. Its hallmarks are its ability to learn, adapt, and remember.
Acquired immunity takes time to develop after first exposure to a new antigen. However afterward, the antigen is remembered, and subsequent responses to that antigen are quicker and more effective than those that occurred after the first exposure.
The white blood cells responsible for acquired immunity are
Typically, an acquired immune response begins when antibodies, produced by B cells (B lymphocytes), encounter an antigen.
Other participants in acquired immunity are
Lymphocytes enable the body to remember antigens and to distinguish self from harmful nonself (including viruses and bacteria). Lymphocytes circulate in the bloodstream and lymphatic system and move into tissues as needed.
The immune system can remember every antigen encountered because after an encounter, some lymphocytes develop into memory cells. These cells live a long time—for years or even decades. When memory cells encounter an antigen for the second time, they recognize it immediately and respond quickly, vigorously, and specifically to that particular antigen. This specific immune response is the reason that people do not contract chickenpox or measles more than once and that vaccination can prevent certain disorders.
Lymphocytes may be T cells or B cells. T cells and B cells work together to destroy invaders.
T cells develop from stem cells in the bone marrow, then travel to an organ in the chest called the thymus. There, they learn how to distinguish self from nonself antigens so that they do not attack the body's own tissues. Normally, only the T cells that learn to ignore the body's own antigens (self-antigens) are allowed to mature and leave the thymus.
T cells can potentially recognize an almost limitless number of different antigens.
Mature T cells are stored in secondary lymphoid organs (lymph nodes, spleen, tonsils, appendix, and Peyer patches in the small intestine). These cells circulate in the bloodstream and the lymphatic system. After they first encounter an infected or abnormal cell, they are activated and search for those particular cells.
Usually, to be activated, T cells require the help of another immune cell, which breaks antigens into fragments (called antigen processing) and then presents antigen from the infected or abnormal cell to the T cell. The T cell then multiplies and specializes into different types of T cells. These types include
Killer (cytotoxic) T cells attach to antigens on infected or abnormal (for example, cancerous) cells. Killer T cells then kill these cells by making holes in their cell membrane and injecting enzymes into the cells.
Helper T cells help other immune cells. Some helper T cells help B cells produce antibodies against foreign antigens. Others help activate killer T cells to kill infected or abnormal cells or help activate macrophages, enabling them to ingest infected or abnormal cells more efficiently.
Suppressor (regulatory) T cells produce substances that help end the immune response or sometimes prevent certain harmful responses from occurring.
When T cells initially encounter an antigen, most of them perform their designated function, but some of them develop into memory cells, which remember the antigen and respond to it more vigorously when they encounter it again.
Sometimes T cells—for reasons that are not completely understood—do not distinguish self from nonself. This malfunction can result in an autoimmune disorder, in which the body attacks its own tissues.
B cells are formed in the bone marrow. B cells have particular sites (receptors) on their surface where antigens can attach. B cells can learn to recognize an almost limitless number of different antigens.
The main purpose of B cells is to produce antibodies, which tag an antigen for attack or directly neutralize it. B cells can also present antigen to T cells, which then become activated.
The B-cell response to antigens has two stages:
Primary immune response: When B cells first encounter an antigen, the antigen attaches to a receptor, stimulating the B cells. Some B cells change into memory cells, which remember that specific antigen, and others change into plasma cells. Helper T cells help B cells in this process. Plasma cells produce antibodies that are specific to the antigen that stimulated their production. After the first encounter with an antigen, production of enough of the specific antibody takes several days. Thus, the primary immune response is slow.
Secondary immune response: But thereafter, whenever B cells encounter the antigen again, memory B cells very rapidly recognize the antigen, multiply, change into plasma cells, and produce antibodies. This response is quick and very effective.
Dendritic cells reside in the skin, lymph nodes, and tissues throughout the body. Most dendritic cells are antigen-presenting cells. That is, they ingest, process, and present antigens, enabling helper T cells to recognize the antigen. Dendritic cells present antigen fragments to T cells in the lymph nodes.
Another type of dendritic cell, the follicular dendritic cell, presents unprocessed (intact) antigen that has been linked with antibody (antibody-antigen complex) to B cells. Follicular dendritic cells help B cells respond to an antigen.
After T and B cells are presented with the antigen, they become activated.
When a B cell encounters an antigen, it is stimulated to mature into a plasma cell or a memory B cell. Plasma cells then release antibodies (also called immunoglobulins, or Ig).
Antibodies protect the body in the following ways:
Helping cells ingest antigens (cells that ingest antigens are called phagocytes)
Inactivating toxic substances produced by bacteria
Attacking bacteria and viruses directly
Preventing bacteria and viruses from attaching to and invading cells
Activating the complement system, which has many immune functions
Helping certain cells, such as natural killer cells, kill infected cells or cancer cells
Antibodies are essential for fighting off certain types of bacterial and fungal infections. They can also help fight viruses.
Antibodies attach to the antigen they were formed to recognize and form an immune complex (antibody-antigen complex). The antibody and antigen fit tightly together, like pieces of a jigsaw puzzle. Sometimes an antibody can attach to other antigens if the antigens closely resemble the antigen that the antibody was formed to recognize and attach to.
Basic Y Structure of Antibodies
Each antibody molecule has two parts:
An antibody can switch its constant part and become a different class, but its variable part does not change. Thus, it can always recognize the specific antigen that it was formed to attach to.
This class of antibody is produced when a particular antigen (such as an antigen of an infectious microorganism) is encountered for the first time. The response triggered by the first encounter with an antigen is the primary immune response. IgM then attaches to the antigen, activating the complement system, and thus makes the microorganism easier to ingest.
Normally, IgM is present in the bloodstream but not in the tissues.
IgG, the most prevalent class of antibody, is produced when a particular antigen is encountered again. More antibody is produced in this response (called the secondary immune response) than in the primary immune response. The secondary immune response is also faster and the antibodies produced—mainly IgG—are more effective.
IgG protects against bacteria, viruses, fungi, and toxic substances.
IgG is present in the bloodstream and tissues. It is the only class of antibody that crosses the placenta from mother to fetus. The mother’s IgG protects the fetus and infant until the infant’s immune system can produce its own antibodies.
These antibodies help defend against the invasion of microorganisms through body surfaces lined with a mucous membrane, including those of the nose, eyes, lungs, and digestive tract.
IgA is present in the following:
These antibodies trigger immediate allergic reactions. IgE binds to basophils (a type of white blood cell) in the bloodstream and to mast cells in tissues. When basophils or mast cells with IgE bound to them encounter allergens (antigens that cause allergic reactions), they release substances (such as histamine) that cause inflammation and damage surrounding tissues. Thus, IgE is the only class of antibody that often seems to do more harm than good. However, IgE helps defend against certain parasitic infections that are common in some developing countries.
Small amounts of IgE are present in the bloodstream and mucus of the digestive system. These amounts are higher in people with asthma, hay fever, other allergic disorders, or parasitic infections.