A. Bacterial Diseases

Other bacterial diseases: Dysentery, Plague, Diphtheria.

B. Viral Diseases

C. Protozoan Diseases

D. Helminthic & Fungal Diseases

A. Innate Immunity

Non-specific type of defense, present at the time of birth. It consists of four types of barriers:

1. Physical Barriers: Skin (main barrier), Mucus coating of epithelium lining the respiratory, gastrointestinal and urogenital tracts (trap microbes).
2. Physiological Barriers: Acid in the stomach, saliva in the mouth, tears from eyes (all prevent microbial growth).
3. Cellular Barriers: WBCs like PMNL-neutrophils (Polymorpho-nuclear leukocytes), Monocytes, Natural Killer (NK) cells (type of lymphocytes) in the blood/tissues, and Macrophages phagocytose microbes.
4. Cytokine Barriers: Virus-infected cells secrete proteins called Interferons which protect non-infected cells from further viral infection.

B. Acquired Immunity

Pathogen specific. It is characterized by memory. When the body encounters a pathogen for the first time, produces a response called primary response (low intensity). Subsequent encounter with the same pathogen elicits a highly intensified secondary or anamnestic response (based on memory).

Cells involved:

• B-lymphocytes: Produce an army of proteins (antibodies) into blood to fight pathogens.
• T-lymphocytes: Do not secrete antibodies but help B cells produce them.

Types of Immune Responses:

• Humoral Immune Response: Mediated by antibodies in blood (fluid).
• Cell-mediated Immunity (CMI): Mediated by T-lymphocytes. Responsible for Graft Rejection in organ transplants (tissue matching and blood group matching are essential; patient takes immuno-suppressants all life).

C. Active and Passive Immunity

D. Allergies

Exaggerated response of the immune system to certain antigens present in the environment.

• Allergens: Mites in dust, pollens, animal dander, etc.
• Antibody produced: IgE type.
• Chemicals released: Histamine and Serotonin from Mast cells.
• Symptoms: Sneezing, watery eyes, running nose, difficulty in breathing.
• Treatment: Antihistamine, Adrenaline and Steroids reduce symptoms quickly.

E. Auto Immunity

Memory-based acquired immunity evolved in higher vertebrates based on the ability to distinguish foreign organisms from self-cells. Sometimes, due to genetic and other unknown reasons, the body attacks self-cells. This results in damage to the body and is called Auto-immune disease. Example: Rheumatoid arthritis.

Lymphoid Organs

Organs where origin and/or maturation and proliferation of lymphocytes occur.

Primary Lymphoid Organs:

• Bone Marrow: Main lymphoid organ where all blood cells including lymphocytes are produced. B-cells mature here.
• Thymus: Lobed organ near the heart and beneath the breastbone. Large at birth but atrophies with age (puberty). T-cells mature here. Provides micro-environment.

Secondary Lymphoid Organs:

• Provide sites for interaction of lymphocytes with the antigen, which then proliferate to become effector cells.
• Spleen: Large bean-shaped organ. Contains lymphocytes and phagocytes. Acts as a filter of the blood by trapping blood-borne microorganisms. Large reservoir of erythrocytes (Graveyard of RBCs).
• Lymph Nodes: Small solid structures along the lymphatic system. Trap microorganisms entering lymph/tissue fluid. Antigens trapped here activate lymphocytes (immune response).
• MALT (Mucosa-associated lymphoid tissue): Lymphoid tissue located within the lining of major tracts (respiratory, digestive, urogenital). Constitutes about 50% of lymphoid tissue in human body.

A. Both A and R are true and R is the correct explanation of A.

B. Both A and R are true but R is NOT the correct explanation of A.

C. A is true but R is false.

D. A is false but R is true.

A. Cholera and Tetanus

B. Typhoid and Smallpox

C. Tetanus and Mumps

D. Herpes and Influenza

A. Both A and R are true and R is the correct explanation of A.

B. Both A and R are true but R is NOT the correct explanation of A.

C. A is true but R is false.

D. A is false but R is true.

A. Autoimmune response

B. Cell-mediated immune response

C. Hormonal immune response

D. Physiological immune response

A. HIV is enveloped virus containing one molecule of single-stranded RNA and one molecule of reverse transcriptase.

B. HIV is enveloped virus that contains two identical molecules of single-stranded RNA and two molecules of reverse transcriptase.

C. HIV is unenveloped retrovirus.

D. HIV does not escape but attacks the acquired immune response.

A. Both A and R are true and R is the correct explanation of A.

B. Both A and R are true but R is NOT the correct explanation of A.

C. A is true but R is false.

D. A is false but R is true.

A. Food in intestine

B. Blood only

C. Erythrocytes, mucosa and submucosa of colon

D. Mucosa and submucosa of colon only

A. More prone to malaria

B. More prone to typhoid

C. Less prone to malaria

D. Less prone to typhoid

A. Both A and R are true and R is the correct explanation of A.

B. Both A and R are true but R is NOT the correct explanation of A.

C. A is true but R is false.

D. A is false but R is true.

A. Elephantiasis

B. Ascariasis

C. Ringworm

D. Amoebiasis

A. Mutualism between host and parasite

B. Ability of the host to fight the disease causing organisms

C. Ability of the parasite to survive within a host

D. A fatal disease

A. These are produced in thyroid

B. There are three main types - cytotoxic T-cells, helper T-cells and suppressor T-cells

C. These originate in lymphoid tissues

D. They scavenge damaged cells and cellular debris

A. Both A and R are true and R is the correct explanation of A.

B. Both A and R are true but R is NOT the correct explanation of A.

C. A is true but R is false.

D. A is false but R is true.

A. Acetylation of morphine

B. Methylation of morphine

C. Carboxylation of morphine

D. Nitration of morphine

A. Within 15 days of sexual contact with an infected person

B. When the infected retro virus enters host cells

C. When HIV damages large number of helper T-Lymphocytes

D. When the viral DNA is produced by reverse transcriptase

A. Both A and R are true and R is the correct explanation of A.

B. Both A and R are true but R is NOT the correct explanation of A.

C. A is true but R is false.

D. A is false but R is true.

A. They compete with normal cells for vital nutrients.

B. They do not remain confined in the area of formation.

C. They divide in an uncontrolled manner.

D. They show contact inhibition.

A. Opium

B. Alcohol

C. Tobacco (Chewing)

D. Cocaine

A. Trophozoite

B. Sporozoite

C. Female gametocyte

D. Male gametocyte

A. Both A and R are true and R is the correct explanation of A.

B. Both A and R are true but R is NOT the correct explanation of A.

C. A is true but R is false.

D. A is false but R is true.

A wide range of organisms belonging to bacteria, viruses, fungi, protozoans, helminths, etc., could cause diseases in man. Such disease-causing organisms are called Pathogens. Most parasites are therefore pathogens as they cause harm to the host by living in (or on) them. They have to adapt to life within the environment of the host (e.g., survive stomach acid).

Salmonella typhi is a pathogenic bacterium which causes typhoid fever in human beings. These pathogens generally enter the small intestine through food and water contaminated with them and migrate to other organs through blood. Sustained high fever (39° to 40°C), weakness, stomach pain, constipation, headache and loss of appetite are some of the common symptoms.

A classic case in medicine, Mary Mallon, nicknamed Typhoid Mary, is worth mentioning here. She was a cook by profession and was a typhoid carrier who continued to spread typhoid for several years through the food she prepared. This highlights the importance of hygiene and screening of food handlers in disease prevention.

Typhoid fever is confirmed by the Widal test. It is a serological test that detects the presence of antibodies against Salmonella typhi in the patient's serum. It is based on the antigen-antibody agglutination reaction. Though widely used, it has limitations in sensitivity and specificity compared to modern culture methods.

Bacteria like Streptococcus pneumoniae and Haemophilus influenzae are responsible for the disease pneumonia in humans which infects the alveoli (air filled sacs) of the lungs. As a result of the infection, the alveoli get filled with fluid leading to severe problems in respiration, reducing gas exchange efficiency.

The symptoms of pneumonia include fever, chills, cough and headache. In severe cases, the lips and finger nails may turn gray to bluish in colour due to hypoxia (lack of oxygen). A healthy person acquires the infection by inhaling the droplets/aerosols released by an infected person or even by sharing glasses and utensils.

Many viruses also cause diseases in human beings. Rhino viruses represent one such group of viruses which cause one of the most infectious human ailments – the common cold. They infect the nose and respiratory passage but not the lungs. The common cold is characterised by nasal congestion and discharge, sore throat, hoarseness, cough, headache, tiredness, etc.

Some of the human diseases are caused by protozoans too. You might have heard about malaria, a disease man has been fighting since many years. Plasmodium, a tiny protozoan is responsible for this disease. Different species of Plasmodium (P. vivax, P. malariae and P. falciparum) are responsible for different types of malaria.

Of the different types of malaria, malignant malaria caused by Plasmodium falciparum is the most serious one and can even be fatal. It causes cerebral malaria where the parasite infected RBCs block the capillaries of the brain. Early diagnosis and treatment with artemisinin-based combination therapies are crucial for survival.

Plasmodium enters the human body as sporozoites (infectious form) through the bite of infected female Anopheles mosquito. These sporozoites are formed in the salivary glands of the mosquito. Once injected into the blood, they travel to the liver where they undergo asexual reproduction before attacking RBCs.

The parasites reproduce asexually in RBCs, bursting the RBCs and releasing more parasites. The rupture of RBCs is associated with release of a toxic substance, haemozoin, which is responsible for the chill and high fever recurring every 3 to 4 days. This cyclic fever is the hallmark symptom of malaria.

Sexual stages (gametocytes) develop in the red blood cells of the human host. The female gametocytes are larger than the male ones. When a female Anopheles mosquito bites an infected person, she takes up these gametocytes with the blood meal. Further development (fertilization) takes place in the mosquito's gut.

Entamoeba histolytica is a protozoan parasite in the large intestine of human which causes amoebiasis (amoebic dysentery). It feeds on red blood cells and damages the intestinal wall using enzymes (histolysin). Trophozoite is the active feeding stage, while cysts are the infective stage passed in stool.

Houseflies act as mechanical carriers and serve to transmit the parasite from faeces of infected person to food and food products, thereby contaminating them. Drinking water and food contaminated by the faecal matter are the main source of infection. This is a classic example of feco-oral transmission.

Ascaris, the common round worm is an intestinal parasite belonging to Aschelminthes. It causes Ascariasis. Symptoms include internal bleeding, muscular pain, fever, anemia and blockage of the intestinal passage. The eggs of the parasite are excreted along with the faeces of infected persons which contaminate soil, water, plants, etc.

Wuchereria (W. bancrofti and W. malayi), the filarial worms cause a slowly developing chronic inflammation of the organs in which they live for many years, usually the lymphatic vessels of the lower limbs. The disease is called elephantiasis or filariasis. The genital organs are also often affected, resulting in gross deformities.

The pathogens of filariasis are transmitted to a healthy person through the bite by the female mosquito vectors. Specifically, Culex mosquitoes are the primary vectors for Wuchereria bancrofti. The microfilariae circulate in the peripheral blood at night (nocturnal periodicity) to be picked up by the mosquito.

Many fungi belonging to the genera Microsporum, Trichophyton and Epidermophyton are responsible for ringworms which is one of the most common infectious diseases in man. Appearance of dry, scaly lesions on various parts of the body such as skin, nails and scalp are the main symptoms of the disease.

Everyday we are exposed to large number of infectious agents. However, only a few of these exposures result in disease. Why? This is due to the fact that the human body has the ability to fight the disease-causing organisms. This overall ability of the host to fight the disease-causing organisms is called immunity.

Innate immunity is non-specific type of defense, that is present at the time of birth. It acts as the first line of defense. It consists of four types of barriers: Physical (skin), Physiological (acid, tears), Cellular (WBCs), and Cytokine (interferons) barriers that prevent entry or survival of pathogens.

Skin on our body is the main barrier which prevents entry of the micro-organisms. Mucus coating of the epithelium lining the respiratory, gastrointestinal and urogenital tracts also help in trapping microbes entering our body. These barriers physically block the entry of potential pathogens into the internal tissues.

Certain types of leukocytes (WBC) of our body like polymorpho-nuclear leukocytes (PMNL-neutrophils) and monocytes and natural killer (type of lymphocytes) in the blood as well as macrophages in tissues can phagocytose and destroy microbes. This forms the cellular barrier of innate immunity.

Virus-infected cells secrete proteins called interferons which protect non-infected cells from further viral infection. They are a part of cytokine barriers. Interferons do not kill the virus directly but induce an antiviral state in neighboring cells, preventing the virus from replicating and spreading.

Acquired immunity is pathogen specific. It is characterized by memory. This means that our body when it encounters a pathogen for the first time produces a response called primary response. Subsequent encounter with the same pathogen elicits a highly intensified secondary or anamnestic response, due to immunological memory.

The primary and secondary immune responses are carried out with the help of two special types of lymphocytes present in our blood, i.e., B-lymphocytes and T-lymphocytes. B-lymphocytes produce an army of proteins (antibodies) in response to pathogens. T-cells themselves do not secrete antibodies but help B cells produce them.

Each antibody molecule has four peptide chains, two small called light chains and two longer called heavy chains. Hence, an antibody is represented as H2L2. Different types of antibodies are produced in our body. IgA, IgM, IgE, IgG are some of them. They are Y-shaped molecules with antigen binding sites.

Because these antibodies are found in the blood, the response is also called as humoral immune response. It is one of the two types of our acquired immune response. It is mainly effective against extracellular pathogens acting in body fluids (humors) like blood and lymph, neutralizing viruses and bacteria.

The second type is called cell-mediated immunity or CMI. The T-lymphocytes mediate CMI. Very often, when some human organs like heart, eye, liver, kidney fail to function satisfactorily, transplantation is the only remedy. CMI is responsible for the graft rejection, hence tissue matching is crucial.

When a host is exposed to antigens, which may be in the form of living or dead microbes or other proteins, antibodies are produced in the host body. This type of immunity is called active immunity. It is slow and takes time to give its full effective response, but it provides long-lasting immunological memory.

When ready-made antibodies are directly given to protect the body against foreign agents, it is called passive immunity. For example, the yellowish fluid colostrum secreted by mother during the initial days of lactation has abundant antibodies (IgA) to protect the infant. It provides immediate but short-lived protection.

The yellowish fluid colostrum secreted by mother during the initial days of lactation has abundant antibodies (IgA) to protect the infant. The foetus also receives some antibodies from their mother, through the placenta during pregnancy. These are examples of natural passive immunity.

The principle of immunization or vaccination is based on the property of 'memory' of the immune system. In vaccination, a preparation of antigenic proteins of pathogen or inactivated/weakened pathogen (vaccine) is introduced into the body. This generates B and T cells that recognize and kill microbial intruders on subsequent exposure.

Recombinant DNA technology has allowed the production of antigenic polypeptides of pathogens in bacteria or yeast. Vaccines produced using this approach allow large scale production and hence greater availability for immunization, e.g., hepatitis B vaccine produced from yeast.

The exaggerated response of the immune system to certain antigens present in the environment is called allergy. The substances to which such an immune response is produced are called allergens. The antibodies produced to these are of IgE type. Common allergens include mites in dust, pollens, animal dander, etc.

Allergy is due to the release of chemicals like histamine and serotonin from the mast cells. Symptoms include sneezing, watery eyes, running nose and difficulty in breathing. Drugs like anti-histamine, adrenalin and steroids quickly reduce the symptoms of allergy. Modern lifestyle has lowered immunity and more people suffer from allergies.

Sometimes, due to genetic and other unknown reasons, the body attacks self-cells. This results in damage to the body and is called auto-immune disease. Rheumatoid arthritis which affects many people in our society is an auto-immune disease. It represents a failure of the immune system to distinguish self from non-self.

The human immune system consists of lymphoid organs, tissues, cells and antibodies. The primary lymphoid organs are bone marrow and thymus where immature lymphocytes differentiate into antigen-sensitive lymphocytes. After maturation the lymphocytes migrate to secondary lymphoid organs.

The bone marrow is the main lymphoid organ where all blood cells including lymphocytes are produced. It provides the micro-environment for the development and maturation of B-lymphocytes. Stem cells in the bone marrow constantly divide to produce precursors for red and white blood cells.

The thymus is a lobed organ located near the heart and beneath the breastbone. It is quite large at the time of birth but keeps reducing in size with age and by the time puberty is attained it reduces to a very small size. It provides the micro-environment for the development and maturation of T-lymphocytes.

The spleen is a large bean-shaped organ. It mainly contains lymphocytes and phagocytes. It acts as a filter of the blood by trapping blood-borne microorganisms. Spleen also has a large reservoir of erythrocytes. It is a secondary lymphoid organ where lymphocytes interact with antigens.

There is lymphoid tissue also located within the lining of the major tracts (respiratory, digestive and urogenital tracts) called mucosa-associated lymphoid tissue (MALT). It constitutes about 50 per cent of the lymphoid tissue in human body. It protects mucosal surfaces from pathogens.

AIDS (Acquired Immuno Deficiency Syndrome) is caused by the Human Immunodeficiency Virus (HIV), a member of a group of viruses called retrovirus, which have an envelope enclosing the RNA genome. AIDS was first reported in 1981. It destroys the immune system, leaving the patient vulnerable to opportunistic infections.

After getting into the body of the person, the virus enters into macrophages where RNA genome of the virus replicates to form viral DNA with the help of the enzyme reverse transcriptase. This viral DNA gets incorporated into host cell's DNA and directs the infected cells to produce virus particles. The macrophages continue to produce virus.

HIV enters into helper T-lymphocytes (TH), replicates and produces progeny viruses. The progeny viruses released in the blood attack other helper T-lymphocytes. This is repeated leading to a progressive decrease in the number of helper T-lymphocytes. This drastic reduction in TH cells causes the severe immunodeficiency.

In our body, cell growth and differentiation is highly controlled and regulated. In cancer cells, there is breakdown of these regulatory mechanisms. Normal cells show a property called contact inhibition by virtue of which contact with other cells inhibits their uncontrolled growth. Cancer cells appear to have lost this property.

Cells sloughed from malignant tumors reach distant sites through blood, and wherever they get lodged in the body, they start a new tumor there. This property called metastasis is the most feared property of malignant tumors. It makes treatment difficult as the cancer spreads to vital organs.

Opioids are the drugs which bind to specific opioid receptors present in our central nervous system and gastrointestinal tract. Heroin, commonly called smack is chemically diacetylmorphine which is a white, odourless, bitter crystalline compound. It is obtained by acetylation of morphine, extracted from the latex of poppy plant Papaver somniferum.

Cannabinoids are a group of chemicals, which interact with cannabinoid receptors present principally in the brain. Natural cannabinoids are obtained from the inflorescences of the plant Cannabis sativa. The flower tops, leaves and the resin of cannabis plant are used in various combinations to produce marijuana, hashish, charas and ganja.

Coca alkaloid or cocaine is obtained from coca plant Erythroxylum coca, native to South America. It interferes with the transport of the neuro-transmitter dopamine. Cocaine, commonly called coke or crack is usually snorted. It has a potent stimulating action on central nervous system, producing a sense of euphoria and increased energy.

Addiction is a psychological attachment to certain effects – such as euphoria and a temporary feeling of well-being – associated with drugs and alcohol. With repeated use of drugs, the tolerance level of the receptors present in our body increases. Consequently the receptors respond only to higher doses of drugs or alcohol leading to greater intake and addiction.