Chapter 7: Human Health and Diseases

Introduction

Health, for a long time, was considered as a state of body and mind where there was a balance of certain ‘humors’. It is now defined as a state of complete physical, mental, and social well-being. Diseases can be broadly grouped into infectious and non-infectious. Diseases which are easily transmitted from one person to another are called infectious diseases (e.g., AIDS). Among non-infectious diseases, cancer is the major cause of death.

Common Diseases in Humans

A wide range of organisms belonging to bacteria, viruses, fungi, protozoans, and helminths could cause diseases in man. Such disease-causing organisms are called pathogens.

• Typhoid: Caused by pathogenic bacterium Salmonella typhi. Spreads through contaminated food and water. Confirmed by the Widal test.
• Pneumonia: Caused by Streptococcus pneumoniae and Haemophilus influenzae. Infects the alveoli of the lungs.
• Common Cold: Caused by a group of viruses called Rhinoviruses. Infects the nose and respiratory passage but not the lungs.
• Malaria: Caused by Plasmodium (Protozoa). P. falciparum causes the most serious, malignant malaria. The human infection cycle starts when a female Anopheles mosquito bites a human. The parasite forms sporozoites, which is the infective stage.
• Amoebiasis: Caused by Entamoeba histolytica (Protozoa) in the large intestine. Houseflies act as mechanical carriers.
• Ascariasis: Caused by Ascaris lumbricoides (Common round worm).
• Ringworms: Fungal diseases caused by genera like Microsporum, Trichophyton, and Epidermophyton.

Immunity

Every day, we are exposed to a large number of infectious agents. However, only a few of these exposures result in disease because our body defends itself from most foreign agents. This overall ability is called immunity.

Innate Immunity

Non-specific type of defence present at the time of birth. Accomplished by providing different types of barriers:

1. Physical barriers: Skin, mucus coating.
2. Physiological barriers: Acid in stomach, saliva in mouth, tears from eyes.
3. Cellular barriers: Leukocytes (WBCs) like PMNL-neutrophils, monocytes, and macrophages.
4. Cytokine barriers: Virus-infected cells secrete proteins called interferons which protect non-infected cells from further viral infection.

Acquired Immunity

Pathogen-specific, characterized by memory. Mediated by B-lymphocytes (producing antibodies – Humoral immune response) and T-lymphocytes (Cell-mediated immunity). An antibody molecule consists of four peptide chains (2 light, 2 heavy) representing \(H_2 L_2\). Types include IgA, IgM, IgE, IgG.

• Active Immunity: When a host is exposed to antigens and antibodies are produced in the host’s body. E.g., natural infection or vaccination.
• Passive Immunity: When ready-made antibodies are directly given to protect the body against foreign agents (E.g., Colostrum having IgA, anti-tetanus shots).

Allergies

The exaggerated response of the immune system to certain antigens present in the environment is called allergy. The substances are called allergens. Mediated largely by IgE antibodies and chemicals like histamine from mast cells.

Autoimmunity

Due to genetic or unknown reasons, the body attacks self-cells, resulting in damage to the body. An example is Rheumatoid arthritis.

AIDS (Acquired Immuno Deficiency Syndrome)

Caused by HIV (Human Immunodeficiency Virus), a retrovirus. It has an RNA genome enveloped with proteins. Once inside a human cell, typically a macrophage or Helper T-cell (\(T_H\)), the viral RNA synthesizes DNA using reverse transcriptase. Viral DNA incorporates into host DNA, producing virus particles. Since it destroys \(T_H\) cells, immunity drops catastrophically, leaving the patient prone to massive infections.

Cancer

Cancer causes normal cell mechanisms to break down. Cancer cells lose the property of contact inhibition, whereby contact with other cells inhibits their uncontrolled growth. These rapidly dividing masses form tumors (benign or malignant). Malignant tumors invade surrounding tissues and establish secondary tumors far away (metastasis). Causes: Carcinogens (physical like UV/X-rays, chemical like tobacco smoke, biological like oncogenic viruses).

Drugs and Alcohol Abuse

Commonly abused drugs:

• Opioids: Receptors in CNS and GI tracts. E.g., Heroin (smack) is chemically diacetylmorphine, a depressant.
• Cannabinoids: Interact with cannabinoid receptors in the brain. E.g., marijuana, hashish, charas, ganja. Known effects on the cardiovascular system.
• Coca alkaloids: Cocaine. Interferes with the transport of dopamine. Has strong stimulating action on CNS.

Competency Based Questions

Q1. An antibody molecule is represented structurally as \(H_2 L_2\). If an antibody monomer is entirely cleaved using the enzyme papain acting at the hinge region, what are the resulting constituent fragments?

(A) Two Fab fragments and one Fc fragment.
(B) Two heavy chains and two light chains completely separated.
(C) One variable region fragment and one constant region fragment.
(D) Random amino acid sequence chains due to denaturation.

Q2. During a malarial infection cycle, human erythrocytes (RBCs) rupture periodically, releasing a specific toxic substance responsible for the chill and high fever recurring every 3-4 days. Name this substance.

Q3. If 500 units of an anti-venom injection containing pre-formed antibodies are administered to a snakebite victim, graph hypothetically what happens to the systemic concentration of these specific antibodies over the next 120 days compared to an active vaccination.

Q4. A laboratory test uses an enzyme-linked reaction to detect the highly specific interaction of a protein envelope glycoprotein component of the HIV virus (e.g., gp120) with patient antibodies. Identify the diagnostic test and the principle of its function.

Q5. By examining the exponential growth equation \(N_t = N_0 e^{rt}\), explain how the concept of “Loss of Contact Inhibition” physically allows a benign mass of cells to adopt equation characteristics that lead to cancer mortality.