Animals and human beings are continually exposed to various
infectious agents like bacteria, viruses, fungi and parasites. It has long been
noticed that survivors of certain diseases, e.g., measles, are not attacked by
the same disease again. Clearly, these people have become immune to the
The system of animal body, which protects it from various infectious agents and cancer, is called Immune system. A study of the immune system is known as Immunology. This chapter introduces the fundamental concepts of immune system and their use for the improvement of human health and welfare.
The Latin term “Immunis”, meaning “exempt” or “freedom”, gave
rise to the English word immunity. It refers to all the mechanisms used by the
body for protection from environmental agents that are foreign to the body.
These agents may be microorganisms or their products, certain food items,
chemicals, drugs and pollen grains. Immunity is of two types : (a) innate, and
(b) acquired immunity.
A. Innate Immunity (Non-specific):
Innate immunity comprises all those natural defense mechanisms with which an organism is protected from infection. As a strategy, innate immunity consists of various types of barriers that prevent entry of foreign agents into the body. The pathogens that enter into the body, are quickly killed by some components of the immune system. This is the first line of defence in most animals. Innate immunity consists of the following four types of barriers. (IMMUNOLOGY)
1. Anatomical Barriers :
These barriers block the entry of organisms into
the body. The skin and the mucous membrane lining the respiratory and
intestinal as well as the reproductive passages constitute the barriers.
Mucous material entraps foreign microorganisms. The ciliary movements
produced by the epithelial lining cells expel out micro-organisms from the body.
2. Physiological Barriers :
Factors like body temperature, pH and various
body secretions, prevent the growth of pathogenic micro-organisms. For
example, fever response inhibits growth of many pathogens. Acidity of the
stomach contents due to HCl secretion kills ingested micro-organisms.
Lysozyme present in secretions, such as tears and saliva, digest bacterial
cell walls. Certain cells, like WBC, when infected with a virus, respond by
releasing anti viral proteins, called interferons. Interferons, in turn, make
the cells in the vicinity resistant to viral infections. As a result, the concerned
persons exhibit increased resistance to viral infections. (IMMUNOLOGY)
3. Phagocytic Barriers :
Phagocytosis is an important mechanism of innate
immunity. It is performed by leucocytes. In response to pathogenic
infections, the total count of leucocytes will increase sharply. Humans
contain wandering phagocytes that circulate throughout the body. The most
important phagocytes are the macrophages and the neutrophils.
Macrophages are large irregular-shaped cells that engulf microbes, viruses
and cellular debris. In response to an infection, monocytes are liberated at the site of infection. These monocytes get converted into macrophages. These
cells are provided with bacteriolytic enzymes and free radicals, which
destroy the pathogens. (IMMUNOLOGY)
4. Inflammatory Barriers :
Usually an infection or tissue injury results in
redness and swelling, along with pain and production of heat that may result
in fever. The above phenomenon is known as inflammatory response. This
response occurs due to release of chemical alarm signals, notably
histamine, serotonin and prostaglandins, by the damaged mast cells.
At the site of inflammation there may be leakage of vascular fluid, which
contains serum proteins with antibacterial activity. Further, there is an influx
of phagocytic cells into the affected area. These responses inhibit and
destroy the invading microorganisms. (IMMUNOLOGY)
Besides the phagocytes, natural killer cells (NK cells) (T Lymphocytes) kill virus-infected cells and some tumour cells of the body by creating perforin-lined pores in the plasma membrane of the target cells. These pores allow entry of water into the target cell, which then swells and bursts.
Related Topics in Zoology:
- Microbiology Introduction and History of Medical Microbiology
- Pasteur, Koch, Lister
- Structure of Viruses
- Viral genetics
- Virus Culture
- Viral Diseases
- Bacteria Structure Culture
- Bacterial Genetics
- Bacterial Diseases
- Protozoan microbiology
- Pathogenecity of Microorganisms
- Antimicrobial Resistance
- Antibiotics and Chemotherapy
- AIDS – HIV