Human Health

Diseases can be broadly grouped into **infectious** (easily transmitted from one person to another) and **non-infectious**. Among non-infectious diseases, cancer is the major cause of death. Among infectious diseases, AIDS is fatal. --- ### AIDS (Acquired Immuno Deficiency Syndrome) - AIDS was **first reported in 1981** and has killed more than 25 million persons worldwide - AIDS 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** - 'Acquired' = not congenital; 'Syndrome' = group of symptoms **Transmission of HIV-infection:** 1. Sexual contact with infected person 2. Transfusion of contaminated blood and blood products 3. Sharing infected needles (intravenous drug abusers) 4. From infected mother to her child through placenta **High-risk groups:** Individuals with multiple sexual partners; drug addicts taking drugs intravenously; individuals requiring repeated blood transfusions; children born to HIV-infected mother. **Important:** HIV/AIDS is NOT spread by mere touch or physical contact — it spreads **only through body fluids**. **Mechanism of HIV infection:** - After entering the body, 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 infected cells to produce virus particles - Macrophages continue to produce virus and act like a **HIV factory** - Simultaneously, HIV enters into **helper T-lymphocytes (T_H)**, replicates and produces progeny viruses - Progeny viruses released in blood attack other helper T-lymphocytes → **progressive decrease in the number of helper T-lymphocytes** in the infected person - The person suffers from bouts of fever, diarrhoea and weight loss - Due to decrease in helper T-lymphocytes, the person starts suffering from infections that could have been otherwise overcome (opportunistic infections) — especially **Mycobacterium** (bacteria), viruses, fungi, and even parasites like **Toxoplasma** **Time gap:** Between infection and appearance of AIDS symptoms: from a few months to **many years (usually 5-10 years)**. **Diagnosis:** Widely used diagnostic test: **ELISA (Enzyme Linked Immuno-Sorbent Assay)** **Treatment:** Anti-retroviral drugs are only partially effective — can only **prolong the life** of the patient but **cannot prevent death**, which is inevitable. **Prevention (NACO and WHO initiatives):** - Making blood (from blood banks) safe from HIV - Ensuring use of only disposable needles and syringes in public and private hospitals and clinics - Free distribution of condoms - Controlling drug abuse - Advocating safe sex - Promoting regular check-ups for HIV in susceptible populations - **NACO (National AIDS Control Organisation)** and NGOs educate people about AIDS --- ### Cancer Cancer is one of the most dreaded diseases and a major cause of death globally. More than a million Indians suffer from cancer annually. **Normal cells vs. Cancer cells:** - In normal cells: cell growth and differentiation is highly controlled - Normal cells show **contact inhibition** — contact with other cells inhibits their uncontrolled growth - **Cancer cells have lost contact inhibition** → continue to divide giving rise to masses of cells called **tumours** **Tumour types:** - **Benign tumours:** Normally remain confined to their original location; do not spread to other parts; cause little damage - **Malignant tumours:** Mass of proliferating cells called neoplastic or tumour cells; grow very rapidly; invade and damage surrounding normal tissues; cells starve normal cells by competing for vital nutrients **Metastasis:** Cells sloughed from malignant tumours reach distant sites through blood; wherever they get lodged, they start a new tumour there. **Metastasis is the most feared property of malignant tumours.** **Causes of cancer (Carcinogens):** - **Physical carcinogens:** Ionising radiations like X-rays and gamma rays; non-ionising radiations like UV — cause DNA damage → neoplastic transformation - **Chemical carcinogens:** Chemical carcinogens present in **tobacco smoke** have been identified as a major cause of lung cancer - **Biological carcinogens:** Cancer-causing viruses called **oncogenic viruses** have genes called **viral oncogenes**. Several genes called **cellular oncogenes (c-onc) or proto-oncogenes** have been identified in normal cells which when activated under certain conditions could lead to oncogenic transformation **Detection and Diagnosis:** - **Biopsy and histopathological studies:** Piece of suspected tissue cut into thin sections, stained and examined under microscope by a pathologist - Blood and bone marrow tests for increased cell counts (in case of leukaemias) - **Radiography** (X-rays), **CT (Computed Tomography)**, **MRI (Magnetic Resonance Imaging)** — useful for detecting cancers of internal organs - CT: uses X-rays to generate 3D image of internals - MRI: uses strong magnetic fields and non-ionising radiations - Antibodies against cancer-specific antigens are also used for detection of certain cancers - PCR can detect mutations in genes in suspected cancer patients **Treatment:** - **Surgery:** removal of tumour - **Radiation therapy (Radiotherapy):** tumour cells are irradiated lethally, protecting surrounding normal tissues - **Chemotherapy:** several chemotherapeutic drugs to kill cancerous cells; specific for particular tumours; side effects include hair loss, anaemia - Most cancers treated by **combination of surgery, radiotherapy and chemotherapy** - **Immunotherapy:** patients given substances called **biological response modifiers** such as **α-interferon (alpha-interferon)** which activate their immune system and help in destroying the tumour --- ### Common Infectious Diseases **Bacterial diseases:** - **Typhoid:** Caused by Salmonella typhi; enters via contaminated food and water; sustained high fever (39°-40°C), weakness, stomach pain; confirmed by **Widal test** - **Pneumonia:** Caused by Streptococcus pneumoniae and Haemophilus influenzae; infects alveoli; fever, chills, cough, headache **Viral diseases:** - **Common cold:** Caused by rhinoviruses; infects nose and respiratory passage but not lungs; lasts 3-7 days **Protozoan diseases:** - **Malaria:** Caused by Plasmodium (P. vivax, P. malariae, P. falciparum). Most serious: P. falciparum (malignant malaria). Transmitted by female Anopheles mosquito. Parasite requires two hosts — human and mosquito. Toxic substance haemozoin responsible for chill and high fever recurring every three to four days - **Amoebiasis:** Caused by Entamoeba histolytica; houseflies act as mechanical carriers **Helminthic diseases:** - **Ascariasis:** Caused by Ascaris (common roundworm) — intestinal parasite; internal bleeding, muscular pain, fever, anaemia - **Filariasis (Elephantiasis):** Caused by Wuchereria (W. bancrofti and W. malayi); transmitted by female mosquito vectors; chronic inflammation of lymphatic vessels of lower limbs ---

The ability of the host to fight the disease-causing organisms conferred by the immune system is called **immunity**. Immunity is of two types: (i) Innate immunity and (ii) Acquired immunity. --- ### Innate Immunity **Innate immunity** is a non-specific type of defence present at the time of birth. It provides different types of barriers to the entry of foreign agents: 1. **Physical barriers:** Skin on our body (main barrier); mucus coating of epithelium lining respiratory, gastrointestinal, and urogenital tracts traps microbes 2. **Physiological barriers:** Acid in the stomach, saliva in the mouth, tears from eyes — all prevent microbial growth 3. **Cellular barriers:** Polymorpho-nuclear leukocytes (PMNL-neutrophils), monocytes, natural killer (type of lymphocytes) in blood and macrophages in tissues — can phagocytose and destroy microbes 4. **Cytokine barriers:** Virus-infected cells secrete proteins called **interferons** which protect non-infected cells from further viral infection --- ### Acquired Immunity **Acquired immunity** is pathogen-specific and characterised by **memory**. - First encounter with pathogen: **primary response** — low intensity - Subsequent encounter with same pathogen: **secondary (anamnestic) response** — highly intensified This is due to the body's memory of the first encounter. Carried out by **two special types of lymphocytes:** **B-lymphocytes:** Produce antibodies (proteins) in response to pathogens into blood. Each antibody molecule has four peptide chains — two small **light chains** and two longer **heavy chains** → represented as H₂L₂. Different types: IgA, IgM, IgE, IgG. Because antibodies are found in blood, the response is called **humoral immune response** (antibody-mediated immunity). **T-lymphocytes:** Do not secrete antibodies but **help B cells** produce them. T-lymphocytes mediate **cell-mediated immunity (CMI)**. CMI is responsible for graft rejection during organ transplant. --- ### Active and Passive Immunity **Active immunity:** When a host is exposed to antigens (living or dead microbes or other proteins) → antibodies are produced in the host body. Active immunity is **slow** and takes time to give its full effective response. - Achieved by: injecting microbes repeatedly during immunisation OR infectious organisms gaining access during natural infection **Passive immunity:** When ready-made antibodies are directly given to protect the body against foreign agents. - Example: **Colostrum** (yellowish fluid secreted by mother during initial days of lactation) has abundant antibodies **(IgA)** to protect the infant — a form of natural passive immunity - The foetus also receives some antibodies from mother through the placenta during pregnancy - In tetanus: pre-formed antibodies or antitoxin (preparation containing antibodies to the toxin) directly injected — this type of immunisation is called **passive immunisation** - Also in snake bites: injection given to patients contains pre-formed antibodies against snake venom --- ### Vaccination and Immunisation The principle of immunisation/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. The antibodies produced would neutralise the pathogenic agents during actual infection. Vaccines also generate **memory B-cells and T-cells** that recognise the pathogen quickly on subsequent exposure and overwhelm the invaders with massive production of antibodies. **Recombinant DNA technology and vaccines:** Allows production of antigenic polypeptides of pathogen in bacteria or yeast. Example: **Hepatitis B vaccine produced from yeast.** --- ### Allergies and Auto-Immunity **Allergy:** The exaggerated response of the immune system to certain antigens present in the environment is called allergy. The substances causing such immune responses are called **allergens** (e.g., mites in dust, pollens, animal dander). Antibodies produced are of **IgE type**. Allergy is due to release of chemicals like **histamine and serotonin** from the mast cells. Treatment: anti-histamine, adrenalin, steroids. **Auto-immunity:** Sometimes the body attacks self-cells. This results in damage called **auto-immune disease**. Example: **Rheumatoid arthritis** is an auto-immune disease. --- ### Lymphoid Organs - **Primary lymphoid organs:** Bone marrow and thymus — where immature lymphocytes differentiate into antigen-sensitive lymphocytes - **Secondary lymphoid organs:** Spleen, lymph nodes, tonsils, Peyer's patches of small intestine, appendix — sites for interaction of lymphocytes with antigen, leading to proliferation into effector cells - **Thymus:** Lobed organ near heart, beneath breastbone; large at birth but reduces with age; provides micro-environment for development and maturation of T-lymphocytes - **Spleen:** Large bean-shaped organ; contains lymphocytes and phagocytes; filters blood by trapping blood-borne micro-organisms; large reservoir of erythrocytes - **MALT (Mucosal-Associated Lymphoid Tissue):** Lymphoid tissue within lining of major tracts (respiratory, digestive, urogenital tracts) — constitutes about **50 per cent** of lymphoid tissue in human body ---