Chapter 14. BASICS OF ANTIGEN-ANTIBODY UPSC Notes | EduRev

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14. BASICS OF ANTIGEN-ANTIBODY

ANTIGEN  

Antigen- A toxin or other foreign substance that induces an immune response in the body, especially the production of antibodies. In immunology, an antigen is a substance that evokes the production of one or more antibodies.

An antibody (Ab), also known as an immunoglobulin (Ig), is a large Y-shaped protein produced by B-cells that is used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, called an antigen. Antibodies are secreted by a type of white blood cell called a plasma cell. Antibodies can occur in two physical forms, a soluble form that is secreted from the cell, and a membrane- bound form that is attached to the surface of a B cell and is referred to as the B cell receptor (BCR). The BCR is only found on the surface of B cells and facilitates the activation of these cells and their subsequent differentiation into either antibody factories called plasma cells, or memory B cells that will survive in the body and remember that same antigen so the B cells can respond faster upon future exposure. In most cases, interaction of the B cell with a T helper cell is necessary to produce full activation of the B cell and, therefore, antibody generation following antigen binding. Soluble antibodies are released into the blood and tissue fluids, as well as many secretions to continue to survey for invading microorganisms.

Antibodies are glycoproteins belonging to the immunoglobulin superfamily; the terms antibody and immunoglobulin are often used interchangeably.

An antigen is a substance that evokes the production of one or more antibodies. Each antibody binds to a specific antigen by way of an interaction similar to the fit between a lock and a key. The substance may be from the external environment or formed within the body. The immune system will try to destroy or neutralize any antigen that is recognized as a foreign and potentially harmful invader. The term originally came from antibody generator and was a molecule that binds specifically to an antibody, but the term now also refers to any molecule or molecular fragment that can be bound by a major histocompatibility complex (MHC) and presented to a T-cell receptor. "Self" antigens are usually tolerated by the immune system, whereas "non-self" antigens can be identified as invaders and can be attacked by the immune system.

An immunogen is a specific type of antigen. An immunogenic is a substance that is able to provoke an adaptive immune response if injected on its own. An immunogenic is able to induce an immune response, whereas an antigen is able to combine with the products of an immune response once they are made. Hapten is a small molecule that cannot induce an immune response by itself. It needs to be attached to a large carrier molecule such as protein. The overlapping concepts of immunogenicity and antigenicity are, therefore, subtly different.

According to a current textbook:

Immunogenicity is the ability to induce a humoral and/or cell-mediated immune response

Antigenicity is the ability to combine specifically with the final products of the immune response (i.e. secreted antibodies and/or surface receptors on T-cells). Although all molecules that have the property of immunogenicity also have the property of antigenicity, the reverse is not true.

 

Miscellaneous Facts:

  1. Prokaryotes have no definite membrane bound nucleus. A cell wall is present only in plants & not in animal cells. Plasma membrane is composed of phospholipids. Enclosed within the plasma membrane is the cytosol in which are embedded the various organelles.
  2. Endoplasmic Reticulum is a network of flattened membranes forming tunnels & generally have ribosome attached (Rough ER) or otherwise (Smooth ER). Involved in protein & macro-molecule synthesis. Golgi complex is another system of membranes generally observed in the form of flattened sacs. It primarily acts as a processing centre for complex macromolecules like glycoproteins.
  3. Chloroplasts are centres of photosynthetic activity & found only in photosynthetic cells. They are bound by a double membrane, enclosing another system of membranes, the lamellae or thylakoids & the matrix (Stroma). Chlorophylls & carotenoids are localized in the thylakoids. Mitochondria have double membrane.
  4. Peroxisomes function to get rid the cell of toxic substances & are found in high number in liver. Glyoxysomes contain oxidative enzymes including those for fat metabolism.
  5. Each ribosome is made up of RNAs & proteins. Lysosomes are bags of hydrolysing enzymes. A vacuole is surrounded by tonoplast. Animal cells have few small vacuoles or none at all. Centrioles help in organizing the mitotic spindle are generally absent in plant cells.
  6. The individual molecules with their associated proteins are called chromosomes. These chromosomes consist of two chromatids. The two chromatids intersect at a knot like structure called centromere.
  7. The stages of mitosis are prophase, metaphase, anaphase & telophase. In mitosis the chromatids are separated & hence the numbers of chromosomes remain the same.
  8. Meiosis involves tow divisions, meiosis I & meiosis II. The prophase I of Meiosis I is divided into five sub stages: leptotene, zygote NE, pachytene, diplotene & diakenesis. Then comes metaphase I, anaphase I, telophase I, prophase II, metaphase II, anaphase II & telophase II.
  9. The chiasmata or crossover formation takes place during the pachytene stage. The distinctive feature of Anaphase I of meiosis I is that it involves the separation of the whole chromosomes & not chromatids as in case of mitosis. The second meiotic division is almost identical to the mitotic division except that A particular virus has either DNA or RNA never both which is protected by a protein coat called capsid.
  10. Plants consist of three types of Simple tissue. The first, called parenchyma, is found throughout the plant and is living and capable of cell division at maturity. The cells of parenchyma tissue carry out many specialized physiological functions—for example, photosynthesis, storage, secretion, and wound healing. They also occur in the xylem and phloem tissues. Collenchyma, the second type of ground tissue, is also living at maturity and is made up of cells with unevenly thickened primary cell walls. Collenchyma tissue is pliable and functions as support tissue in young, growing portions of plants. Sclerenchyma tissue, the third type, consists of cells that lack protoplasts at maturity and that have thick secondary walls usually containing lignin. Sclerenchyma tissue is important in supporting and strengthening those portions of plants that have finished growing.
  11. Complex tissues are xylem & phloem. Xylem is meant for the transport of water & mineral elements. Phloem is meant for the transport of food material from the leaves to the consumption organs. The epidermis is the outermost layer & is primarily protective in function. The epidermis of leaves is interrupted by fine pores called stomata which are guarded by a pair of guard cells. The guard cells together with the adjacent epidermal cells constitute the stomata.
  12. Eight essential amino acids are needed to maintain health in humans: leucine, isoleucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
  13. The process of development of an embryo, & subsequently the whole plant from an unfertilized egg is called parthenogenesis.
  14. There are three types of RNA: mRNA (carries message from the DNA for protein synthesis), rRNA (ribosomes attached) & tRNA (transfer of amino acids to the site of protein synthesis). All the three types of RNA are synthesized from DNA, one of whose stand act as a template. This process is called transcription. The process of decoding the transcribed information is called translation during which the genetic code is converted to language of proteins.
  15. A nucleotide is a chemical compound that consists of a heterocyclic base, a sugar, and one or more phosphate groups. In the most common nucleotides the base is a derivative of purine or pyrimidine, and the sugar is pentose - deoxyribose or ribose. Nucleotides are the structural units of RNA & DNA. Groups of three nucleotides, known as codons, code for the 20 amino acids that form the foundation of proteins. The triplet code results in 64 codons. UAA, UAG & UGA do not code for any amino acid & Are called non sense codons. They are in fact the stop signals. AUG is likewise the start signal but it also codes for methionine if it falls in the middle. The code is universal & is identical in all organisms.
  16. The indirect flow of information from DNA to proteins is called the Central Dogma. However HIV virus has a reverse transcriptase mechanism of RNA to DNA synthesis.
  17. Adenine pairs with thymine & guanine with cytosine. Uracil is found in RNA only & Thiamine in DNA only. Except for the mature RBCs all human cells contain the complete genome. The human genome is estimated to contain 20,000-25,000 genes & 3.2 billion chemical nucleotides bases. Less than 2% of genome codes for proteins.
  18. The outermost layer of the kidney is called the cortex. Beneath the cortex lies the medulla. Nephrons, extend between the cortex and the medulla. The primary structure of nephrons is the glomerulus, a network of extremely thin blood vessels called capillaries. The glomerulus is contained in a cuplike structure called Bowman’s capsule, from which extends a narrow vessel, called the renal tubule. This tube twists and turns until it drains into a collecting tubule that carries urine toward the renal pelvis. Part of the renal tubule, called Henley’s loop, becomes extremely narrow, extending down away from Bowman’s capsule and then back up again in a U shape.
  19. The right side of the heart is responsible for pumping oxygen-poor blood to the lungs, while the left side of the heart is responsible for pumping oxygen-rich blood to the body. The oxygen-poor blood feeds into two large veins, the superior vena cava and inferior vena cava, which empty into the right atrium of the heart. The right atrium conducts blood to the right ventricle, and the right ventricle pumps blood into the pulmonary artery. The pulmonary artery carries the blood to the lungs, where it picks up a fresh supply of oxygen and eliminates carbon dioxide. The blood, now oxygen-rich, returns to the heart through the pulmonary veins, which empty into the left atrium. Blood passes from the left atrium into the left ventricle, from where it is pumped out of the heart into the aorta.
  20. Contraction of ventricles is known as systole & the muscular relaxation that follows is diastole. The right atrioventricular valve is formed from three flaps of tissue and is called the tricuspid valve, while the left atrioventricular valve has two flaps and is called the bicuspid or mitral valve. The other two valves are located between the ventricles and arteries. They are called semilunar valves. The right semilunar valve is called the pulmonary valve. The left semilunar valve, between the left ventricle and aorta, is called the aortic valve. The first heart sound Lob is caused by the closure of a trio-ventricular vales & Dubby closure of aortic & pulmonary valves.
  21. Diabetes insipid us (DI) is characterized by excretion of large amounts of severely diluted urine due to deficiency of vasopressin & is not characterized by high sugar level. Diabetes Miletus is caused by deficiency of insulin.
  22. About 55 percent of the blood is composed of a liquid known as plasma. The rest of the blood is made of three major types of cells: RBCs (erythrocytes – 44% by volume), WBCs (leukocytes 1%), and platelets (thrombocytes).
  23. There are several varieties of white blood cells, including neutrophils, monocytes, and lymphocytes. A foreign substance which, when introduced into the body, stimulates the production of an antibody is called an antigen.
  24. In classification, the sequence is kingdom, phylum, class, order, family, genus & species. For e.g. In Homo sapiens, Homo is the genus & sapiens is the species.
  25. Whittaker’s classification which divides the organisms into five kingdoms is the most popular. The kingdoms are: Monera (blue green algae), Protista (Diatoms), Fungi, Plantae & Animalia.
  26. Amphibians & reptiles have 3 chambered hearts. However crocodiles have four chambered heart.
  27. Interferon refers to any of a group of antiviral proteins produced by animals.
  28. Lathyrism - prevalent in some areas of India and Africa, where grass pea (Lathyrus sativus) is consumed (used as an adulterant in flour). If used for 2-6 months it causes spastic paralysis of the legs.
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