NCERT Summary Summary of Biology- 4 - Science & Technology for UPSC CSE

Table of Contents
1. Lymphocytes and the Immune System
2. Antigen Recognition, Self and Non-self
3. Blood Groups, Rh Factor, and Transfusion Compatibility
4. Organ Transplants and Histocompatibility
5. Body Defences and Phagocytes
6. Vaccines
7. Blood: The Vital Fluid
8. Respiratory System
9. Circulatory System
10. Diseases of the Heart and Cardiovascular System
11. The Vascular Routes: Pulmonary and Systemic Circulation
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Lymphocytes and the Immune System

Lymphocytes are a type of white blood cell that develop from stem cells in the bone marrow by mitosis. Some lymphocytes migrate to the thymus to mature into T cells, which circulate in the blood and are concentrated in lymphoid organs such as lymph nodes and the spleen. Other lymphocytes mature in the bone marrow as B cells and then enter the circulatory and lymphatic systems. B cells produce antibodies.

Types of Immunity

• Antibody-mediated (humoral) immunity: regulated by B cells and the antibodies they produce; defends primarily against extracellular pathogens such as many bacteria and viruses circulating in body fluids.
• Cell-mediated immunity: regulated by T cells; defends against infected body cells (intracellular pathogens), parasites, fungi, protozoa and also recognises and destroys cancerous cells.

Stages of Antibody-mediated (Humoral) Immunity

1. Detection of antigen.
2. Activation of helper T cells and other antigen-presenting cells.
3. Activation and proliferation of B cells and production of antibodies.

Each stage is carried out by specific cell types described below.

Cells and their roles

• Macrophages (antigen-presenting cells): these phagocytic white blood cells engulf foreign microbes or particles, digest them and display small antigen fragments on their surface bound to major histocompatibility complex (MHC) molecules. This antigen presentation is essential to activate T cells.
• Helper T cells: helper T cells (CD4+ T cells) become activated when they recognise antigen fragments presented on MHC class II molecules of antigen-presenting cells. Activated helper T cells signal and activate B cells and other immune cells.
• B cells: when activated by antigen and helper T cells, B cells proliferate and differentiate into plasma cells that secrete large quantities of antibodies, and into memory B cells that persist for months or years to provide faster responses on re-exposure.
• Cytotoxic (killer) T cells: these T cells (CD8+ T cells) recognise infected body cells that display foreign antigens on MHC class I molecules and destroy those infected cells by releasing proteins that create pores in the target cell membrane, leading to cell death; they also contribute to rejection of transplanted tissue.
• Suppressor (regulatory) T cells: moderate and terminate immune responses, acting as an "off switch" to prevent excessive immune activity.
• Memory T cells: long-lived cells that remain in the body to enable a faster, stronger response if the same antigen is encountered again.

Antibodies (Immunoglobulins)

Antibodies are proteins (immunoglobulins) produced by plasma cells. They bind specific antigens in a lock-and-key fashion to form antigen-antibody complexes. There are five major classes of immunoglobulins: IgG, IgA, IgM, IgD and IgE.

An antibody molecule is Y-shaped and consists of two identical long polypeptide chains (heavy chains) and two identical short polypeptide chains (light chains). The tips of the Y form the antigen-binding sites (variable regions) that are specific for each antigenic determinant (epitope). Antibody functions include:

• Recognition and specific binding to antigens.
• Neutralisation or inactivation of pathogens or toxins.
• Opsonisation to promote phagocytosis.
• Activation of the complement system to destroy pathogens.

Antigen Recognition, Self and Non-self

The immune system distinguishes self from non-self by recognising chemical markers (antigens) on cell surfaces. All cells carry surface molecules that the immune system uses to identify them. When lymphocytes encounter non-self antigens they mount a specific immune response. Antibodies are produced by B lymphocytes (through plasma cells); T lymphocytes do not produce antibodies but mediate cellular responses and help B cells.

Blood Groups, Rh Factor, and Transfusion Compatibility

Many antigens occur on the surface of red blood cells; these determine blood groups. The most important clinically are the ABO system and the Rh factor.

ABO Blood Groups

• Type A: A antigen on red cells; anti-B antibodies in plasma.
• Type B: B antigen on red cells; anti-A antibodies in plasma.
• Type AB: both A and B antigens on red cells; no anti-A or anti-B antibodies in plasma; individuals can receive blood of any ABO type (universal recipient for ABO).
• Type O: neither A nor B antigens on red cells; both anti-A and anti-B antibodies in plasma; individuals can donate to any ABO type (universal donor for ABO) but can receive only type O.

Rh Factor and Haemolytic Disease of the Newborn (HDN)

The Rh system includes the D antigen; individuals are Rh+ if they have the D antigen and Rh- if they lack it. If an Rh- mother carries an Rh+ fetus, fetal Rh+ red blood cells may enter the maternal circulation at delivery (or during some pregnancies or procedures), causing the mother to produce anti-Rh antibodies. In a subsequent Rh+ pregnancy these maternal antibodies can cross the placenta and destroy fetal red cells, causing haemolytic disease of the newborn (HDN).

To prevent HDN, Rh- mothers are given anti-Rh immunoglobulin (Rho(D) immune globulin) during and/or after the first pregnancy with an Rh+ fetus; this prevents maternal sensitisation and reduces risk to future Rh+ fetuses.

Organ Transplants and Histocompatibility

Successful organ transplants and skin grafts require close matching of histocompatibility antigens expressed on all cells. Genes on chromosome 6 encode the human leukocyte antigen (HLA) complex, critical to graft acceptance. The set of HLA alleles on one chromosome is called a haplotype. Because many alleles exist, it is unlikely for unrelated individuals to share identical haplotypes.

• Identical twins are a 100% HLA match.
• Best donor preference: identical twin > sibling > parent > unrelated donor.
• Chance of an unrelated donor matching varies and has been estimated as about 1 in 100,000-200,000; matches are more difficult across different ethnic or racial groups.

Body Defences and Phagocytes

Phagocytes are specialised cells that ingest and destroy microbes and foreign particles (phagein = to eat; cyte = cell). They are widespread but concentrated in liver, spleen and bone marrow.

• Monocytes in the blood are circulating precursors of tissue macrophages.
• Specific (acquired) immunity has two branches: humoral immunity (antibody-mediated) and cellular immunity (T cell-mediated).
• Lymphoid organs that produce lymphocytes include bone marrow, thymus, lymph nodes, spleen and specialised patches in the wall of the small intestine (e.g., Peyer's patches).
• The two main lymphocyte types are B lymphocytes (humoral immunity) and T lymphocytes (cellular immunity).
• Antibodies are proteins that belong to the gamma-globulin fraction of plasma proteins; they are also called immunoglobulins.
• Two historical ideas on antibody synthesis are worth noting: early "instructive" ideas (now obsolete) suggested the antigen shaped the antibody; the accepted modern explanation is the clonal selection theory (proposed by F. Macfarlane Burnet), which states that each lymphocyte bears receptors of a single specificity and antigen selects the appropriate clone to expand.

Vaccines

• Hepatitis B vaccine: a primary course typically requires three doses; a common schedule is first dose, second dose after one month, and a third dose several months later (six months after the first dose is a common recommendation for completion of the primary series).
• Oral typhoid vaccine: an oral capsule vaccine is available; brand names vary (the name 'Typhoral' is used for some oral typhoid formulations).

Blood: The Vital Fluid

Blood is a suspension of cells in a liquid called plasma. When spread thinly, blood reveals different cell types: red blood cells (erythrocytes), white blood cells (leucocytes) and platelets (thrombocytes).

Plasma

Plasma is a straw-coloured fluid, about 90% water, with dissolved salts and proteins. The principal salt is sodium chloride. Plasma proteins include fibrinogen (essential for clot formation) and globulins (many of which contribute to defence and transport).

Red Blood Cells (Erythrocytes)

• In adult mammals, red blood cells lack a nucleus and mitochondria and contain haemoglobin, an iron-containing protein that transports oxygen.
• Normal haemoglobin concentration is about 12-15 g per 100 ml of blood in healthy adults; a sustained decrease is called anaemia.
• Clinical signs of anaemia include pallor of mucous membranes (for example, the palate or conjunctiva).
• Average life span of a human red cell is about four months (≈120 days); they are produced in the bone marrow.

White Blood Cells (Leucocytes)

White blood cells are fewer in number than red cells (roughly one WBC to several hundred RBCs). They have nuclei, lack haemoglobin and some can move to engulf particles by phagocytosis.

Major types of white blood cells include:

• Neutrophils
• Eosinophils
• Basophils
• Lymphocytes
• Monocytes

Platelets (Thrombocytes)

• Platelets are small, anuclear cell fragments derived from bone marrow megakaryocytes.
• They contribute to haemostasis (stoppage of bleeding) by forming a platelet plug and by releasing chemical mediators and clotting factors (for example, serotonin, thromboxane A2 and platelet factors) that promote coagulation and conversion of fibrinogen to fibrin.

Respiratory System

Respiration in Single-celled and Simple Animals

Single-celled organisms exchange gases directly across their cell membrane. Because diffusion is slow over long distances, single cells and very small organisms remain small. Simple animals that lack specialised respiratory surfaces have flattened, tubular or thin body shapes to increase surface area for gas exchange.

Respiration in Multicellular Animals

Large animals cannot rely on diffusion across the outer surface and have developed specialised respiratory surfaces (gills, lungs, tracheae) that increase surface area and are kept thin and moist to allow oxygen and carbon dioxide to dissolve and diffuse across epithelial cells into blood or haemolymph.

Principles of Respiration

1. Movement of an oxygen-containing medium so it contacts a moist membrane overlying blood vessels.
2. Diffusion of oxygen from the medium into the blood or transport fluid.
3. Transport of oxygen to the tissues and cells of the body.
4. Diffusion of oxygen from the blood into cells.
5. Carbon dioxide follows a reverse path from cells to the external medium.

Alveoli and Gas Exchange

Alveoli are clusters of thin-walled air sacs at the termini of the bronchial tree. They are richly supplied with capillaries and provide a very large surface area for gas exchange. An adult human has on the order of 300-700 million alveoli (a commonly cited estimate is about 600 million), which gives an enormous exchange surface for oxygen and carbon dioxide.

• Oxygen diffuses from the alveoli into the blood; carbon dioxide diffuses from the blood into the alveoli to be exhaled.

Circulatory System

Circulatory Systems in Simple Organisms

Single-celled organisms use their cell surface for exchange. Even simple multicellular animals like sponges move seawater through their bodies to transport nutrients and wastes. Small animals such as hydra and planaria rely on diffusion across their skin; this limits their maximum size and led to the evolution of specialised circulatory systems in larger organisms.

Circulatory Systems in Multicellular Organisms

Multicellular animals use circulatory systems to distribute oxygen, nutrients, hormones and to remove carbon dioxide and metabolic wastes. Main components of a circulatory system are:

• Blood: a connective tissue of plasma and cells.
• Heart: a muscular pump that propels blood.
• Blood vessels: arteries, capillaries and veins that deliver blood to tissues.

Vertebrate Cardiovascular System

The vertebrate cardiovascular system contains a heart and an extensive network of blood vessels. Blood enters the heart via the upper chambers, the atria, passes through valves into the lower pumping chambers, the ventricles, and is forced out of the heart by ventricular contraction.

The heart muscle is composed of specialised cardiac muscle cells. Valves ensure unidirectional flow of blood through the heart.

Arteries, Capillaries and Veins

• Arteries carry blood away from the heart and have thick, elastic walls with smooth muscle that allow them to withstand and smooth out the high pressure of blood pumped by the heart. The largest systemic artery is the aorta.
• The pulmonary artery is exceptional in that it carries oxygen-poor blood from the heart to the lungs for oxygenation.
• Arterioles are small arteries that deliver blood to capillary beds.
• Capillaries are one cell layer thick and are the sites of exchange of gases, nutrients and wastes between blood and tissues. Some capillaries have small pores to allow passage of materials and certain cells.
• Control of flow into capillary beds is achieved by smooth muscle sphincters under nervous and local control.
• Extensive networks of capillaries in the human body are estimated at tens of thousands of miles in total length (commonly cited estimates are in the range of 50,000-60,000 miles).
• Blood leaving capillary beds collects into venules, then into veins that return blood to the heart. Veins, except pulmonary veins, typically carry oxygen-poor blood. Veins have valves to prevent backflow and rely on skeletal muscle contractions to assist venous return because venous pressure is low.

Blood Pressure and Its Control

Blood pressure is the force exerted by circulating blood on vessel walls and is commonly measured in millimetres of mercury (mm Hg). A typical healthy young adult blood pressure is recorded as systolic/diastolic and expressed as about 120/80 mm Hg (ventricular systole / ventricular diastole).

As blood flows away from the heart, pressure falls. Pressure receptors (baroreceptors) in arteries and heart chambers send nerve signals to the medulla oblongata in the brain, which adjusts heart rate, vascular resistance and other parameters to regulate blood pressure.

Comparative notes from some animals illustrate how circulation varies: for example, humans have higher arterial pressures than some invertebrates (a cited comparison is human 120/80 vs lobster ~12/1), and circulation times vary (human systemic circulation on the order of seconds; in some animals circulation is considerably slower).

Diseases of the Heart and Cardiovascular System

• Heart attack (myocardial infarction): coronary arteries supply oxygenated blood to cardiac muscle. Narrowing or blockage of coronary arteries, often by atherosclerotic plaque (lipids and cholesterol accumulation), reduces blood flow and can cause death of cardiac muscle cells. Angina pectoris is chest pain that indicates reduced oxygen supply to the heart and may precede a heart attack. Since adult cardiac muscle cells have very limited ability to divide, damaged heart tissue is not readily replaced.
• Hypertension (high blood pressure): a chronic condition in which arterial pressure is persistently elevated. A common clinical threshold is sustained pressures above 140/90 mm Hg. Causes are often multifactorial (genetic predisposition, lifestyle factors such as obesity, salt intake, stress and smoking). Hypertension is treatable with lifestyle measures and medication.

The Vascular Routes: Pulmonary and Systemic Circulation

There are two principal circuits in vertebrates:

• Pulmonary circuit: moves blood to and from the lungs for gas exchange (pulmonary arteries → lungs → pulmonary veins).
• Systemic circuit: moves blood between the heart and the rest of the body (aorta and systemic arteries → tissues → systemic veins → venae cavae → heart).

Some animals have specialised portal systems in which blood flows from one capillary bed to another capillary bed (for example, from the digestive tract to the liver). Fish pump blood through gills for gas exchange before it passes to the rest of the body; mammals pump blood to the lungs and back to the heart before systemic distribution. Blood normally flows in one direction through a healthy circulatory system.