Hello, and welcome to today's biology lesson. We are going to explore Aids to Health. Together, we will discover how our body protects itself from diseases, understand the different types of immunity, and learn about the medicines and treatments that help us stay healthy.
Let us begin with a simple question: why do we need to stay healthy? Every one of us wishes to live free from disease. This requires effort at two levels — personal and community. At the personal level, we must keep our bodies clean through washing, bathing, brushing our teeth, exercising regularly, eating proper food, and getting adequate rest. At the community level, we must ensure our surroundings remain clean, with no garbage accumulation or stagnant water where disease-spreading insects can breed.
World Health Day is celebrated every year on April seventh to spread awareness about health and hygiene at all levels of society.
Now, let us dive into the core concept of this chapter: immunity.
Immunity is the body's defence mechanism against diseases. Our bodies are constantly invaded by harmful substances — pollutants, poisonous chemicals, and germs. These invaders can enter our body in four ways: directly through the skin, through the mucous membranes of our eyes, nose, or urinary and genital tracts, through the food and water we consume, and through the air we breathe.
Our body responds in two stages. First, it tries to prevent the entry of these harmful substances. Second, if they do enter, the body fights them to render them harmless.
The defence system operates at two distinct levels. The first is the local defence system, which prevents germs from entering. The second is the immune system, which deals with germs after they have penetrated the body tissues.
Let us examine the local defence system in detail.
This system acts as a barrier at the possible entry points of germs. It includes protective mechanical barriers, mechanisms to expel germs if they enter, germ-killing secretions, and germ-fighting white blood cells.
The protective mechanical barriers include our skin, hair, and mucus. The skin has an outer tough layer made of a protein called keratin, which is almost impermeable to germs. Even though many germs settle on the skin surface from air or contact, washing with soap and water removes them. Any cut or scratch opens a pathway for germs, but blood clotting plugs the wound to block their entry.
Hairs intercept germs before they can reach the skin. The hairs inside your nostrils trap dust containing germs.
Mucus is a slimy secretion from the epithelial lining of various organs. In your nasal passage and windpipe, mucus traps bacteria and prevents their entry into body tissues. The cilia — tiny hair-like structures in your windpipe — then sweep out these trapped bacteria.
If germs do enter, the body has direct methods to expel them. Coughing, sneezing, and vomiting throw out germs or foreign objects from your respiratory and digestive systems. Even diarrhoea helps eliminate germs from an infected digestive tract.
Your body also produces germ-killing secretions. Saliva, sweat, tears, and nasal secretions contain substances that kill germs. Most importantly, hydrochloric acid in your stomach kills germs that enter with food.
Finally, white blood cells stand ready as germ-fighters. Should any microbe enter, these phagocytes — a type of white blood cell — attack them. Through a process called diapedesis, white blood cells squeeze out of blood capillary walls. They then engulf and destroy bacteria through phagocytosis. The pus you see in a wound is actually a mixture of destroyed germs, killed white blood cells, and damaged tissue cells.
The local defence system has three remarkable merits. It starts working instantaneously, it does not depend on previous exposure to infections, and it is effective against a wide range of potentially infectious agents.
Now, what happens when germs breach these barriers? This is where the immune system takes over.
Certain microbes or their poisonous secretions — called toxins — can enter deeper tissues through special mechanisms or through breaches in our protective barriers. At this stage, blood and other body fluids begin fighting these invaders.
These body fluids contain special proteins called antibodies, which react with invading germs, and antitoxins, which react with their poisons to destroy them.
Let us define immunity precisely. Immunity is the capacity of our body to deal with foreign substances — bacteria, viruses, toxins, and others — that enter our body and to render them harmless. Simply put, it provides resistance against disease-causing germs.
Immunity is classified into two main categories: innate immunity and acquired immunity.
Innate immunity, also called natural or native immunity, exists by virtue of our genetic constitutional make-up. It is present without any external stimulation or previous infection. Non-specific innate immunity provides natural resistance to all infections in general — for example, humans do not suffer from plant diseases or certain animal diseases. Specific innate immunity provides natural resistance to particular germs only — for instance, humans are immune to distemper, a highly infectious disease of dogs.
Acquired immunity is resistance to a disease that an individual develops during his or her lifetime. This can happen in two ways: through previous infection, called actively acquired immunity, or through ready-made antibodies supplied from outside, called passively acquired immunity.
Actively acquired immunity develops when your own body responds to a previous infection or to an antigen — a chemical found on the surface of disease-causing germ cells. This can occur naturally through infection, or artificially through vaccination. In both cases, lymphocytes in your body produce antibodies that circulate in blood and lymph to bind and kill microorganisms. They also produce killer cells with specific receptors for foreign antigens. This immunity is usually long-lasting because of memory lymphocytes.
Passively acquired immunity comes not from your own body but from an outside source in the form of ready-made antibodies. Naturally acquired passive immunity occurs when mother's antibodies reach the foetus through the placenta. Artificially acquired passive immunity involves antibodies produced in the blood of a horse or other animal by injecting germs into its body. Antiserum injections prepared from such animals' blood are given to patients — for example, antivenin for snake bites or anti-diphtheria injections.
Let us compare active and passive immunity directly. Active immunity is produced by your own body, while passive immunity is received from outside. Active immunity is induced by infection or contact with immunogens — immunity-producing agents such as vaccines. Passive immunity is provided by ready-made antibodies. Active immunity provides effective and long-lasting protection, whereas passive immunity is less effective and temporary. Finally, active immunity takes time to become effective as your body produces antibodies, while passive immunity works immediately.
Now, let us understand antibodies in greater detail.
Antibodies are special proteins in the blood that act against germs or their secretions. They are proteins belonging to the class of immunoglobulins. Antibodies are produced by specialised lymphocytes on exposure to antigens — chemical substances found on germ cells. These lymphocytes concentrate in lymph nodes, the spleen, and circulating blood and lymph.
Your body can make an unlimited variety of different antibodies. Each antibody is specific — one kind acts against only one particular type of antigen. An antibody recognises its particular antigen, binds to it, renders it harmless, and the body then destroys and eliminates it.
Some people have certain antibodies from birth, giving them natural protection against specific diseases even if germs cross their barriers and escape phagocytes. Immunity produced by antibodies may last for a short period, as in common cold or cholera, or for a longer period, as in smallpox or measles.
Before moving forward, let us briefly mention a condition where the immune system fails: AIDS, or Acquired Immuno Deficiency Syndrome. This disease is caused by HIV, the Human Immunodeficiency Virus. It is transmitted through sexual contact, contaminated syringe needles, and blood transfusions. The virus attacks immune system cells, causing a marked reduction in T-cells of the thymus — the cells that activate lymphocytes to produce immunity. With collapsed immunity, severe infections develop unchecked, ultimately causing death.
Let us now explore vaccination and immunisation — artificial methods of developing immunity.
Vaccination is the practice of artificially introducing germs or germ substances into the body to develop resistance to particular diseases. Scientifically, this is called prophylaxis, and the material introduced is called the vaccine. Vaccines are usually given by injection, though some like polio drops are given orally. Inside the body, the vaccine stimulates white blood cells to produce antibodies against that particular disease.
There are four categories of vaccines. First, killed germs — such as TAB vaccine for typhoid, Salk's vaccine for poliomyelitis, and rabies vaccine. Second, living weakened germs — such as measles vaccine and the freeze-dried BCG vaccine for tuberculosis. BCG stands for Bacillus Calmette-Guérin, named after two French scientists who developed this strain. Third, living fully poisonous germs — such as the smallpox vaccine using cowpox virus, which is similar to smallpox virus but causes only a single pustule rather than multiple pustules. Smallpox vaccinations are no longer given because the disease has been totally eradicated. Fourth, toxoids — inactivated toxins secreted by bacteria, such as those for diphtheria and tetanus. These toxins are made harmless by adding dilute formalin while retaining their capacity to produce antibodies.
Let us define our terms precisely. Vaccination is the introduction of any kind of dead or weakened germs into the body to develop immunity against the respective disease. Immunisation is developing resistance to disease-producing germs or their toxins by introducing killed germs or germ substances to induce the production of specific antibodies.
India follows a National Immunisation Schedule. Between three to twelve months, babies receive three doses of DPT vaccine — for diphtheria, pertussis or whooping cough, and tetanus — plus three doses of oral polio vaccine and BCG vaccine. At nine to fifteen months, they receive one dose of measles vaccine. At eighteen to twenty-four months, booster doses of DPT and polio are given. At five to six years, children receive DT booster and two doses of typhoid TAB vaccine. At ten and sixteen years, tetanus toxoid and typhoid boosters are administered. Pregnant mothers receive tetanus toxoid doses to protect both mother and baby. Since fifteen years of age, COVID-19 vaccines have been included in the schedule.
Now, let us clarify the terms toxin and antitoxin.
A toxin is any poisonous substance produced by animals, plants, or bacteria — such as snake venom, scorpion stings, or poisons released by pathogens growing inside the body. Antitoxin was the name given to chemical substances produced in response to foreign poisonous substances. Today, we use the more general term antibody. An antibody is a blood serum protein produced in response to injected antigens.
When someone actually has a disease like diphtheria, injecting pre-prepared antibodies from another source can help. Antibody-containing serum is obtained from horses or rabbits in which the disease is artificially produced in mild form. This treatment is called passive immunisation.
Let us now distinguish between antiseptics and disinfectants — both important for preventing disease, but used differently.
Antiseptics are mild chemical substances that kill germs when applied on the body. They are in such mild concentration that they cause no harm to skin or body tissues. Examples include lysol, carbolic acid, iodine, benzoic acid, mercurochrome, and boric acid in dilute solution. Certain antibiotic creams also serve as antiseptics. Remember, commercial names like Dettol or Savlon are brand names — it is the active ingredients that actually work as antiseptics.
Disinfectants are strong chemical substances applied to spots and places where germs thrive and multiply. Common examples include cresol, phenol, lysol, forty percent formalin, lime, bordeaux mixture, and DDT. All disinfectants are strong and should never contact human skin. Strong heat and boiling also destroy germs and may be called physical disinfectants.
Deodorants are neither antiseptics nor disinfectants — they only mask bad smells.
To summarise the difference: antiseptics are mild, safe for skin, and applied on the body, while disinfectants are strong, potentially harmful to skin, and applied on surfaces.
Finally, let us explore antibiotics — one of medicine's greatest discoveries.
Antibiotics are chemical substances produced by some microorganisms that can kill or inhibit the growth of other microorganisms. The first antibiotic, penicillin, was discovered in nineteen twenty-eight by Alexander Fleming. Fleming was growing cultures of Staphylococcus bacteria when he noticed that an unwanted mould, Penicillium, had inhibited bacterial growth. Some substance from the mould was killing bacteria — Fleming named it penicillin. Its first human use came in the early nineteen forties, showing remarkable effectiveness against several infections, particularly gonorrhoea, a sexually transmitted disease. The term antibiotics was coined by Selman Waksman in nineteen forty-two.
Since then, many antibiotics have been discovered: streptomycin, chloromycetin, aureomycin, ampicillin, and others. Antibiotics are also used to prevent infections after surgical operations.
The mould that produced penicillin was Penicillium notatum, similar to mould growing on citrus fruits. Commercial production uses a related species, Penicillium chrysogenum, though penicillin is also produced synthetically. Streptomycin comes from a bacterium called Streptomyces. Today, many antibiotics are made synthetically rather than from cultured microorganisms.
Antibiotics have multiple uses: fighting infections in medicine, preserving fresh meat and fish as food preservatives, treating animal feed to prevent internal infections, and controlling plant pathogens.
How do antibiotics kill germs? Penicillin interferes with cell wall formation, preventing bacteria from growing and multiplying. Streptomycin binds to bacterial ribosomes, stopping protein synthesis and preventing bacterial growth.
Before antibiotics, another group of medicines called sulphonamides was developed. In nineteen ten, a drug called salvarsan based on an arsenic compound was produced to kill syphilis and sleeping sickness germs, though it often poisoned patients too. In the nineteen thirties, sulphonamides were discovered — sulphadiazine and sulphanilamide are examples. These synthetic drugs interfere with bacterial metabolism, killing the bacteria. Today, sulphonamides are rarely used, and only in combination with antibiotics for certain conditions.
Let us now recap the key takeaways from this chapter.
First, your body has a two-level defence system: local defence barriers that prevent germ entry, and the immune system that fights germs after they enter.
Second, immunity can be innate — present from birth through genetic makeup — or acquired — developed during your lifetime through active or passive means.
Third, active immunity is produced by your own body and provides long-lasting protection, while passive immunity uses ready-made antibodies from outside sources and gives immediate but temporary protection.
Fourth, vaccination and immunisation artificially introduce weakened or killed germs to stimulate your body to produce antibodies and develop immunity.
Fifth, antiseptics are mild germ-killers safe for body use, while disinfectants are strong germ-killers for surfaces that can harm skin.
Sixth, antibiotics are substances produced by microorganisms that kill or inhibit other microorganisms, revolutionising medicine since penicillin's discovery.
Understanding these aids to health empowers you to make informed decisions about your wellbeing and appreciate the remarkable ways science helps us combat disease. Stay curious, stay healthy, and keep exploring the fascinating world of biology. Thank you for listening, and I look forward to our next lesson together.