This series of articles comes with an English-Brazilian Portuguese downloadable glossary of terms used in immunology with the English terms explained (in English) and translated into Portuguese. You can download it now. The file size is 160K and the format is RTF (opens in most word processors).
What is Immunity?
Historically the term immunity has meant protection against disease and more specifically, infectious diseases. The various cells and proteins responsible for immunity constitute the immune system, and their collective and orchestrated response to the introduction of foreign substances (also called “non-self” substances) is the immune response. Nowadays, we know that the same basic mechanisms of resistance to infections are also involved in the individual’s response to non-infectious foreign substances. Thus, when an individual has a primary contact with a molecule or cell, the immune system will first discriminate if this is a “self” or a “non-self” agent. Under normal conditions, if this substance/cell is the same found in the organism, the immune system will not react and we say that the individual is tolerant to that agent. However, if the agent is recognized as a “non-self” substance/cell it will trigger a specific immune response, in addition to a non-specific one, in an attempt to destroy it. These foreign substances that elicit a specific immune response and react with the product of this response are generically called antigens. The mechanisms that normally protect individuals from infections and eliminate foreign substances are themselves capable of causing tissue injury and disease (e.g., auto-immune diseases, rejection of grafts, allergies) in some situations. Thus, Immunology deals with understanding how the body distinguishes between “self” and “non-self” molecules; the remainder is technical detail...
Innate and Acquired Immunity
Vertebrates present two main types of immunity: innate (also known as natural immunity) and acquired or adaptive immunity.
Innate immunity encompasses the cells and molecules with which an individual is born and it is potentially ever-present, available on short notice and non-specific; also, innate immunity is the first line of defense against foreign cells or substances. The innate immune system provides an immediate, non-lasting resistance which is not improved by repeated infection.
Acquired immunity, on the other hand, is specific to the foreign molecule or cell, thus being an adaptive response to a given “non-self” substance and also presents memory (i.e., the immune system “remembers” a previous encounter with a foreign microbe or molecule, so that subsequent encounters increasingly stimulate defense mechanisms). The immunological memory is the basis of the protective vaccination against infectious diseases. For example, infection or vaccination against smallpox, diphtheria or pertussis produce a persistent immunity following infection or vaccination and the development of “memory lymphocytes,” which in turn will induce a more effective, long-lasting and stronger immune response after a subsequent infection or vaccination.
The innate and specific immune systems consist of a variety of molecules, cells and tissues.
The most important cells are the leukocytes which fall into two broad categories: phagocytes (including macrophages and neutrophyls) and natural killer cells, which belong to the innate immune system, and lymphocytes (specially T lymphocytes), which mediate the adaptive immunity.
The most important soluble factors that mediate the innate immune response are: lysozyme, a complex of substances generically called the complement system and the so called acute-phase proteins (e.g., interferons and C-reactive protein). The main soluble proteins responsible for the acquired immune response are the antibodies.
If the first innate defenses are breached, the specific immune mechanisms are activated and produce a specific reaction to each infectious agent in an attempt to erradicate that agent. Also the specific immune response amplifies the protective mechanisms of natural immunity, thus reinforcing the body’s ability to eliminate the antigenic molecules.
Innate immunity
Most infectious agents encountered by an individual are prevented from entering the body surface by a variety of physical and biochemical barriers, such as the intact skin, mucus, cilia lining the trachea, acidity of the stomach, lysozyme (a protein present in saliva and most secretions which is able to split a bond of some bacterial cell walls) and commensal organisms in the vagina and guts.
If an infectious agent penetrates an epithelial surface, it will meet a second set of barriers: the phagocytes and the natural killer (NK) cells. Phagocytes are able to engulf particles, including many bacteria and fungi species, and destroy them, a process called phagocytosis. The main phagocytic cells are the neutrophyls, the monocytes and the macrophages. NK cells are also leukocytes, which are able to recognize cell surface changes that occur in tumoral cells and in virus infected cells. NK cells are then able to bind to those altered cells and kill them. This type of reaction in which a lymphocyte kills a target cell is called cytotoxicity.
In addition to the phagocytic and NK cells, soluble substances also operate in a coordinated way, to erradicate the infectious agents. These include some molecules referred to as acute-phase proteins, the complement system proteins and interferons, which increase rapidly in numbers during infections.
Acute phase proteins encompass the C-reactive protein, a protein that can bind to pneumococci and other bacteria and promote the activation of some complement-system proteins.
The complement system is a complex of more than twenty serum proteins, whose overall functions are to facilitate phagocytosis by binding to the antigens (a process called opsonization), to control inflammation and to destroy foreign agents through lysis of these cells. The complement proteins interact with each other and with other elements of the innate and specific immune system components.
Interferons (IFNs) are a group of proteins that are important in viral infections. Interferon a and b are produced by cells infected by virus and they act on other cells to induce a state of resistance to viral infection. Another IFN, known as IFN-g, is produced during the specific acquired immunity response phase.
Acquired immunity
When an individual is exposed to a foreign antigen, two basic types of effector mechanisms are normally stimulated. One is mediated by specific molecules, called antibodies. Antibodies are present in the blood and various biological fluids and the antibody-mediated immunity is called humoral immunity. The other type of immune response is effected by cells, mainly by the so called T lymphocytes, and confers a cell-mediated immunity.
Most immune responses involve the activity and interplay of both the humoral and the cell-mediated immune branches of the immune system. Furthermore, the innate and the adaptive immune systems do not act in a totally independent way. The following examples illustrate this: antibodies opsonize infectious agents so that phagocytes recognize and engulf their targets more effectively; activated T lymphocytes produce certain hormones called cytokines and some of these cytokines stimulate phagocytes to destroy infectious agents in a more efficient way; T lymphocytes help the so called B lymphocytes to produce antibodies.
So, the immune system operates as an orchestra in which all musical instruments are important, although in some parts there are some solos, and the conductor is the foreign agent which many times determines which (if any) kind of response(s) will be elicited.
In the next installment of this series we will approach the cells and molecules involved in immunity, how they operate and we will have some words about the deleterious effects that may result from the activation of the immune system.
If you have any questions, comments or suggestions for further topics in the field of immunology or immunology nomenclature, please contact the author at: lsinger@icb.usp.br or at Lsinger@originet.com.br.
|
