In this article we will discuss about the resistance of human body against foreign agents.
While the innate non-specific defense mechanisms are present in the body since birth, the acquired resistance develops in the body only in response to a foreign agent. Such resistance is effective only against the agent which evokes the response and not to other agents. Thus, this property is restricted to a particular person and is effective against a specific agent.
While the innate mechanisms provide the first line of defense, because they are already present when a foreign offender invades, the acquired resistance develops slowly to provide a second but more effective line of defense.
The offending agent provokes the formation of specialized cells and proteins in the body making it competent to fight against the offending agent and remove its harmful effects. This reaction of the body to the foreign agent is known as immune response and the resulting ability developed in the body as immunity.
Another important feature which distinguishes immunity from innate resistance is that some of the specialized cells produced through an immune response persist in the body’s circulatory system as memory cells. These cells can provoke an immune response against the same foreign agent in case it invades the body subsequently.
Such response is quick and stronger than the initial response. In case of the innate resistance no such memory cells are present and they are also unnecessary, because the innate resistance mechanisms are always present in the body and spontaneous in their activity against foreign agents.
The human defense systems are distinguished into two categories — the innate and the acquired, the two systems operate in close association with each other. Also, they often interact with each other through many cellular components and chemical mediators which are common to both the innate and acquired defense systems.
Some of these cells, like Langerhans cells, macrophages, and chemical mediators, like complement, cytokines etc. also take part in the acquired defense systems.
The ultimate aim of both innate and acquired defense mechanisms is to protect the human body from the harmful effects of foreign agents. These agents may be cellular microorganisms, like bacteria, fungi, protozoa etc. or acellular agents, like viruses, or some harmful products of biological origin. In any case, the offending object must be alien or foreign to the body.
The Immune Response:
The immune system has the capacity to distinguish between ‘self and ‘non-self which enables the system to recognise objects as foreign which do not belong to the body. The primary object of the immune system is to destroy anything which enters into the body tissues or to inactivate it, thereby giving protection against the harmful effects of the foreign material.
Such foreign materials include not only pathogenic agents like bacteria, fungi, viruses, protozoa or larger organisms like helminths, but may be large molecules, like those of proteins, polysaccharides, certain lipids and their conjugated products, like lipoproteins, lipopolysaccharides, nucleoproteins etc.
Also foreign tissues, like skin or organ grafts, blood etc., are recognized by the immune system as foreign or non-self objects. Furthermore, certain cells of the body — when they behave abnormally, such as cancer cells — are also recognized by the immune system as non-self and they are destroyed.
This comes under the purview of immune surveillance of the immune system. The recognition of self and non-self is, therefore, of crucial importance in provoking an immune response.
The non-self objects which elicit an immune response are commonly called an antigen. The immune response to such antigens involves production of specific proteins in the body, called antibodies, and specialized cells, called lymphocytes. Both antibodies and the specialized cells attack the objects identified as foreign and remove their harmful effects. This property developed through an immune response is called immunity.
The immune system can recognize the particular antigen on a second encounter. This is known as immunological memory. Immunity is highly specific in that it is targeted only against a particular foreign agent, i.e. antigen, which evokes the immune response and not to any other antigen.
Types of Immunity:
Generally, two types of immunity are recognized — active and passive. Active immunity is acquired by an individual through experience with an antigen. This may happen when a person is attacked by a pathogen and survives. As a consequence, the person develops immunity against that pathogen and can resist a second attack by the same pathogen.
The person has thus acquired active immunity in a natural way. Active immunity may also be acquired artificially when a person is vaccinated with an antigen of a particular pathogen. The vaccine contains an inactivated pathogen or some of its product in a form which can evoke an immune response, but not the actual disease. The person receiving the vaccine develops artificially acquired active immunity.
Immunity may also be acquired passively when a person receives ready-made antibody against an antigen. In this case, the antibody is produced generally in some animal by immunization against a pathogen and the serum containing the antibody (antiserum) is introduced into a person by injection.
The person develops, thereby, a passive acquired immunity against that pathogen. An example of this type of immunity is provided when a person receives an anti-tetanus serum (ATS) or anti-tetanus globulin (Tetglob). This gives immediate protection against tetanus, a disease caused by Clostridium tetani, because the antiserum introduced into the body contains ready-made antibodies active against the pathogen.
Another form of passive immunity develops naturally in the fetus when some antibodies present in the mother pass into it through the placenta. Thus, a new-borne child acquires some immunity from the mother. Some antibodies are also secreted in the milk, specially in the milk immediately after childbirth (colostrum). The sucking baby receives these antibody and acquires passive immunity in a natural way.
In general, active acquired immunity induced either through an attack of a pathogen or through a vaccination has a longer duration, because in such immunity the antibodies and reactive cells of the immune system are continuously generated in the body of the immunized person. In contrast, passive immunity produced by antiserum or passage of maternal antibodies into child is comparatively short lived and lasts so long as the introduced antibodies remain active.
Different types of immunity are summarized in Fig. 10.12:
The Immune System:
Acquired immunity embraces two different but interlinked systems, both of which involve lymphocytes. One of these is known as humoral immunity mediated by a specific type of globulin proteins, called antibodies. The other is cell-mediated immunity involving specialized cells, the T-lymphocytes. Both systems are induced in response to antigen.
Components of the Immune System:
Humoral immunity depends on production of antibodies which constitute a group of glycoproteins belonging to the class y-globulins, known as immunoglobulin’s (Ig). They are produced by the plasma cells which arise from a population of lymphocytes known as B-cells. The B-cells are so called because they originate and undergo maturation in the bone-marrow in adult persons.
The antibodies produced by plasma cells in response to specific antigens are present in blood, lymphatic system and other tissue fluids and are able to react with the specific antigen. Humoral immunity primarily affords protection by such antigen-antibody reaction.
When the antigen is a microbe or a free virus particle, the binding with antibody favours phagocytosis by the phagocytic cells of the body resulting in destruction of the parasite. In case of a virus, binding with antibody results in loss of attachment to the target body cell.
Antibodies also react with soluble antigens, such as microbial toxins and the binding inactivates the toxin by neutralizing its harmful effects. A small proportion of B-cells produced in an immune response circulate in the body as memory cells.
Cell-mediated or cellular immunity involves another kind of lymphocytes, known as T-cells, because these cells — after originating from bone-marrow of adults — migrate to the thymus gland where they undergo maturation.
The T-cells are again of two broad categories:
The T-helper cells (TH) and
Cytotoxic T-cells (CTL).
T-helper cells are so called because they help other cells, like B-cells, to proliferate and differentiate into plasma cells.
The cytotoxic T-cells are mainly involved in destruction of body cells infected with viruses and intracellular microbes. They are also responsible for rejection of foreign tissues, like skin and organ grafts. Abnormal body cells, like cancer cells, are targets of the cytotoxic T-cells. So, the cytotoxic T-cells play the main role in cell-mediated immunity.
Both B-cells and T-cells in their resting stage, i.e. when they are not activated, are similar in morphology and cannot be distinguished under the microscope. They, together with the natural killer cells, belong to the lymphocyte family of agranular leucocytes, possessing a large nucleus occupying the major part of the cytoplasm and measuring on the average 10-20 μm in diameter.
Of the total lymphocytes, about 80% are T-cells, 10% are B-cells and 10% are natural killer cells. When a B-cell is transformed into an antibody-producing plasma cell, it undergoes a morphological change and becomes much larger in size with elaborate intra-cytoplasmic membrane system (endoplasmic reticulum) with attached ribosomes.
The lymphocytes undergo maturation in the primary lymphoid organs, like bone marrow in case of B-cells and thymus in case of T-cells. Then they are trans-located by blood stream to the secondary lymphoid organs, like lymph nodes and spleen. T-cells and B-cells migrate to different parts of lymph nodes.