Immunity:

• Immunity means the body’s defense against disease.

  • Our body is all the time invaded by various harmful substances like pollutants (poisonous chemicals) and germs.

  • They may enter our body in the following four ways:

    • Directly through the skin,

      • Through mucous membranes of the eyes, nose, urinary or genital tracts,

    • Through food or water intake

      • Through the air, we breathe in.

      • Our body system first tends to prevent their entry into the body.

      • Secondly, if somehow, they do enter, the body fights with these so that they cause no harm (rendered harmless).

      • The defense system in our body works at two levels:

• Local Defense System:

  • This is a kind of barrier system that tackles the germs at their possible entry points.

    • It includes:

      • protective mechanical barriers

      • thrown out, if entered

      • germ-killing secretions, and

      • germ-fighting WBCs.

• Immune System:

  • It deals with the germs after they have entered the body tissues.

Local Defense System:

• Protective mechanical barriers: These include

the skin, hair, mucus, etc.

• Skin: Skin has an outer tough layer made of the protein keratin and it is almost impermeable to germs.

  • At any given time, there are lots of germs settled on the outer surface of the skin, which have come through the air or through direct contact with contaminated objects.

  • Even a handshake or a kiss on the cheeks of a baby transfers germs from one individual to another.

  • Washing with soap and water removes the germs.

  • Any scratch or cut in the skin opens the way for germs to get in.

• Should there be any cut in the skin, the clotting of blood plugs the cut ends of blood vessels at the wound to prevent the entry of germs.

• Hairs: Hairs intercept the germ’s journey inwards or up to the skin.

  • Hairs inside the nostrils trap dust containing the germs.

• Mucus: Mucus is a slimy secretion of the epithelial lining of various organs.

  • For example:

    • The mucus secreted in the nasal passage and the windpipe traps many bacteria and prevents their entry into the body tissues.

    • The Cilia of the windpipe throw out the bacteria trapped in the mucus.

• Thrown out, if entered:

  • Coughing, sneezing, and vomiting are direct methods to throw out germs or any foreign unwanted objects which get into the respiratory and digestive systems.

  • Even diarrhea (loose motions) helps in throwing out the germs if the infection persists in the digestive tract.

• Germ-killing secretions:

  • Seller, sweat, tears, and nasal accretions.

    contain germ-killing substances.

  • Hydrochloric acid secreted by the stomach kills germs that gain entry along with the food.

• Germ-fighting white blood cells (WBCs): Should any microbe enter the body systems; the white blood cells (phagocytes) are ready to fight them.

  • They squeeze out of the walls of the blood capillaries (by diapedesis) and engulf the bacteria or the germs and destroy them (phagocytosis).

  • Pus in a wound, for example, is a mixture of destroyed germs, killed WBCs, and damaged tissue cells.

Merits of Local Defense System:

• They start working instantaneously.

  • They are not dependent on previous exposure to infections.

    • They are effective against a wide range of potentially infectious agents.

Immune System:

• The local defense systems are not effective in all cases and circumstances.

  • Certain microbes (germs) or their poisonous secretions (toxins) do enter the deeper tissues and various organ systems, by their special mechanisms of entry or through any breaches in the protective barriers.

    • At this stage, the blood and other body fluids, start fighting against germs or any other unacceptable foreign substance.

    • The body fluids contain special proteins called antibodies which react with the invading germs, and antitoxins which react with their poisons (toxins) to destroy them, and thus they provide protection against disease (immunity)

      • To define more elaborately — Immunity is the “capacity of our body to deal with foreign substances, e.g., bacteria, viruses, toxins, etc. that enter our body and to render them harmless.” or simply, “it provides resistance against disease—causing germs.”

Kinds of Immunity:

• Immunity can be classified into two main categories-innate and acquired immunity and their different subcategories are as follows:

Innate Immunity:

• It is also called natural or native immunity.

  • This immunity is by virtue of genetic constitutional makeup.

    • It is there in the body without any external stimulation or a previous infection.

  • Non-specific innate immunity. A degree of natural resistance to all infections in general.

    • For example, humans do not suffer from the plants’ highly infectious diseases or even certain diseases of animals.

  • Specific innate immunity: This is a natural resistance to a particular kind of germ only.

    • Some races or some individuals do not suffer from certain infectious diseases.

• For example, human beings are immune to a highly infectious disease of dogs (known as ‘Distemper’), which kills about 509 of all infested dogs.

Acquired Immunity:

• Resistance to a disease that an individual acquires during his lifetime.

  • It may be the result of either a previous infection (actively acquired immunity),

    • Example: A person having once suffered from “measles” will not normally suffer from it again, or “ready-made” antibodies supplied from outside (passively acquired immunity),

    • Example: A person bitten by a poisonous snake is given an anti-venin injection (venom: poison) which contains antibodies for the poison that was produced in the body of a horse.

• Actively Acquired Immunity:

  • This is the resistance developed by an individual due to a previous infection or antigen (a chemical found on the surface of the disease-causing germ cell) which enters his body naturally leading to:

    • naturally acquired active immunity) or is introduced artificially, as in vaccinations leading to

    • artificially acquired active immunity.

  • In either case, the body lymphocytes react in two ways:

    • They produce antibodies that freely circulate in the blood & lymph, and which bind to the microorganism to kill it.

    • They produce killer cells carrying specific receptors for foreign antigens found in

      invading germs.

    • The actively acquired immunities are usually long-lasting and carried out through ‘memory’ lymphocytes.

    • Artificially  acquired  Passive  immunity:

      • In this, the antibodies are produced in the blood of a horse or some other animal by injecting germs into its body.

      • Antiserum injections are prepared from the serum (containing antibodies) of such animals’ blood and are injected into the body of the patient, e.g. in the treatment of snake-bite by antivenin or that of a diphtheria patient by anti-diphtheria injections.

      • Haffkine’s Institute in Bombay and another institute at Kasauli are preparing several such anti-sera. Antivenin for treating snake-bite is also based on the same principle.

      • For the sake of simplicity, let us just take the active and passive immunities and summarise the differences between them.

Antibodies:

• These are special chemicals found in the blood which act against germs or their secretions.

  • Some of the characteristics of the antibodies are as follows:

    • The antibodies are proteins (they belong to the class of immunoglobulins).

    • Antibodies are produced by a type of specialized lymphocytes on exposure to antigens (chemical substances found in the germs’ cells).

      • These special lymphocytes particularly concentrate in the lymph nodes and spleen and also in the circulating blood and lymph.

        • Our body can make an unlimited variety of different antibodies.

      • Antibodies are specific which means that one kind of antibody acts against only one particular type of antigen.

        • An antibody recognizes its particular antigen and binds to it rendering it harmless, which is subsequently destroyed and eliminated by the body.

        • Some antibodies are present in the blood of some people from the very birth.

      • Such people having these particular antibodies in their blood do not suffer from those particular diseases even if the germs for them have crossed the barriers and escaped the phagocytes.

        • The immunity produced by the antibodies on exposure to antigens may be either for a short period (as in the common cold, cholera), or for a longer period (as in smallpox, measles, etc.).

Vaccination and Immunization:

• Vaccination is the practice of artificially introducing germs or germ substances into the body for developing resistance to particular diseases.

  • Scientifically, this practice is called prophylaxis and the material introduced into the body is called the vaccine.

    • The vaccine or germ substance is introduced into the body usually by injection and sometimes orally (e.g. polio drops).

      • Inside the body, the vaccine stimulates the WBCs to produce antibodies against germs for that particular disease.

        • The terms “vaccine’ and vaccination” were originally used for vaccination against smallpox, but now these are used in a general sense.

          • Killed germs, such as the TAB vaccine for typhoid, Salk’s vaccine for poliomyelitis, and the vaccine for rabies (dog bite).

            • Living weakened germs, such as the vaccine for measles, and the freeze-dried BCG vaccine for tuberculosis.

            • The full form of BCG is “Bacillus of Calmette and Guerin” after the names of two French workers who developed this strain for vaccination.

          • Living fully poisonous germs, as for smallpox.

            • In this vaccination, a person is inoculated with the cowpox virus which is very similar to the smallpox virus.

          • Cowpox virus causes only a single postule to develop rather than multiple pustules of smallpox all over the body.

            • Vaccination by cowpox vaccine protects from smallpox as well.

            • Smallpox vaccinations are no more given because the disease has been totally eradicated according to present-day records.

              • Toxoids (inactivated toxins secreted by bacteria), as for diphtheria and tetanus.

              • These toxins (poisons) are made harmless by the addition of dilute formalin, yet retain the capacity to produce antibodies (antitoxins).

                • Attempts are being made to develop a vaccine against AIDS also, let us pray we succeed in it.

Antitoxins:

• The terms ‘°Toxin” and “Antitoxin”

  • The toxin is a general term used for any poisonous substance produced by an animal or a plant or a bacterium.

    • Examples: Snake venom, sting poisons of scorpions, insects, etc., or even poisonous chemicals released by pathogens growing inside the body.

      • “Antitoxin” was the name given to any chemical substance produced inside the body in response to the entry of foreign poisonous substances.

      • “Antivenins” for snake bites are produced inside animals like horses. Presently, the more general term “antibody” is used instead of “antitoxin”.

      • Suppose, someone has actually got a disease such as diphtheria, in such a case, injecting pre-prepared antibodies from some other source is helpful.

        • For this, an antibody-containing serum is obtained from the blood of horses, rabbits, etc., in which the disease is artificially produced in a mild form.

        • Treatment with such antibodies is called passive immunization.

          • Haffkine’s Institute in Bombay and another institute at Kasauli are preparing several such anti-sera. Antivenine for treating snake-bite is also based on the same principle.