This resistance (immunity) of epithelial surfaces to invasion is a characteristic which is present from birth.
Hence the immunity means the all capacity of human body to resist almost all types of organisms or toxins that tend to damage the tissue and organs.
1. Innate Immunity (Non specific):
Present from birth
Non specific, acts on many organisms and does not show specificity
Does not become more efficient on subsequent exposure to same organisms.
It is the general defense of body, including the following four mechanisms:
(1) Phagocytosis of invaders by leucocytes and tissue macorphages
(2) Destruction of micro-organisms swallowed with food by the HC1 of gastric juice and by digestive enzymes
(3) Resistance of skin to invading micro-organisms
(4) Destruction of invaders and toxins in the blood itself by certain defensive compounds such as lysozymes, properdin, basic polypetides and naturally occurring antibodies, which are always present in the blood plasma.
This innate immunity makes human body resistant to certain animal diseases such as hog cholera, cattle plague, dysentery, etc.
On the other hand, it makes animal bodies resistant to certain severe human diseases such as poliomyelitis, mumps, measles, syphilis, human cholera, etc.
It is the ability to develop special defence system against powerful invaders and toxins. Acquired immunity also known as adopted immunity may be of humoral or cellular types.
Humoral immunity – Body develops circulatory antibodies made up of globuline protein, e.g.: a, (3, y, capable of attacking invader. These antibodies are produced by B cells, which are also known as B-lymphocytes and antibodies as humoral antibodies.
2. Cellular immunity:
It is cell mediated response i.e. body produces large number of special sensetile cells known as sensetile lymphocyte (i.e. T-lympyocyte). T-lymphocytes also known as T-cells are produced in thymus.
Cellular immunity protects from gradual progressing diseases, e.g. Tuberculosis, cancer, leprosy, AIDS.
Active immunity is resistance induced after contact with foreign antigens, e.g., microorganisms.
This contact may consist of clinical or subclinical infection, immunization with live or killed infectious agents or their antigens, or exposure to microbial products (e.g. toxins and toxoids).
In all these instances, the host actively produces an immune response consisting of antibodies and activated helper and cytotoxic T lymphocytes.
The main advantage of active immunity is that resistance is long-term. Its major disadvantage is its slow onset, especially the primary response.
Passive immunity is resistance based on antibodies preformed in another host. Administration of antibody against diphtheria, tetanus, botulism, etc., makes large amounts of antitoxin immediately available to neutralize the toxins.
Likewise, preformed antibodies to certain viruses (e.g., rabies and hepatitis A and B viruses) can be injected during the incubation period to limit viral multiplication.
The main advantage of passive immunization is the prompt availability of large amount of antibody; disadvantages are the short life-span of these antibodies and possible hypersensitivity reactions if globulins from another species are used.
Passive-active immunity involves giving both preformed antibodies (immune globulins) to provide immediate protection and a vaccine to provide long-term protection.
These preparations should be given at different sites in the body to prevent the antibodies neutralizing the immunogens in the vaccine. This approach is used in the prevention of tetanus, rabies and hepatitis B.
Cells and tissues involved in immunity comprise about 2% of the body weight. These are basically the components of lymphoid organs which are divided into 3 categories. These are
The bone marrow, thymus and fetal liver are the major sites of production of lymphocytes, and the environments where lymphocytes undergo differentiation and proliferation so that they leave as functional effectors cells.
Lymph nodes, spleen, tonsils and Peyer’s patches provide an environment in which lymphocytes can interact with each other and with antigen because there are phagocytic macrophages, antigen presenting cells, mature T and B lymphocytes collected together,
In addition to the organized lymphoid tissues, there are other sites, particularly the mucosae, where many immunocytes are dispersed between other cells, for example within gut epithelium and lamina propria.