(a) Functions of immunoglobulins (10 marks) (b) ABO system of blood groups and its basis, and erythroblastosis foetalis (8+2=10…

(a) Functions of immunoglobulins

Immunoglobulins are heterotetrameric glycoproteins, each with two identical heavy chains and two light chains, forming Fab antigen-binding and Fc effector regions. The Fab region determines antigen specificity, while the Fc region determines effector functions such as complement binding and Fc-receptor interaction. IgG is the major serum antibody; it opsonizes microbes, neutralizes toxins, activates complement, and crosses the placenta via FcRn to give passive neonatal immunity. IgM is a pentamer, appears in the primary response, and is a potent complement activator. IgA, especially secretory IgA, protects mucosal surfaces in saliva, tears, milk and gut. IgE binds mast cells and basophils, mediating immediate hypersensitivity and defence against helminths through eosinophils and mast-cell degranulation. IgD mainly serves as a membrane-bound B-cell receptor, participating in B-cell maturation and activation.

(b) ABO system and erythroblastosis foetalis

The ABO system (Landsteiner, 1900) is based on carbohydrate antigens on red-cell membranes. Its genetic basis is a single gene on chromosome 9 with three multiple alleles: Iᴬ and Iᴮ are codominant and both dominant over i, giving six genotypes and four phenotypes (A: IᴬIᴬ or Iᴬi; B: IᴮIᴮ or Iᴮi; AB: IᴬIᴮ; O: ii). The H antigen, made by FUT1, is modified by A or B glycosyltransferases; type A has A antigen, type B has B antigen, type AB both, type O only H. Naturally occurring IgM anti-A or anti-B antibodies occur in plasma of individuals lacking the corresponding antigen. The Bombay phenotype (hh) lacks H antigen, so A/B antigens cannot be expressed and such persons make anti-H, making transfusion special. Erythroblastosis foetalis mainly follows Rh incompatibility: an Rh-negative mother sensitized by Rh-positive fetal cells makes IgG anti-D, which crosses the placenta in later pregnancies and haemolyses fetal Rh-positive erythrocytes. Anti-D immunoglobulin (RhoGAM), given to the Rh-negative mother at about 28 weeks and within 72 hours of delivery, prevents maternal sensitization. ABO incompatibility, usually an O mother with an A or B baby, can also cause a milder haemolytic disease because some anti-A/anti-B antibodies are IgG.

(c) Olfactory pathways and physiology

Olfaction begins in the olfactory epithelium of the nasal cavity, where bipolar olfactory receptor neurons bear cilia in mucus and Bowman’s glands secrete mucus. Odorant molecules bind G-protein-coupled odorant receptors on the cilia (a family of about 400 functional genes in humans, discovered by Axel and Buck, Nobel Prize 2004); each neuron expresses one receptor type. The olfactory G protein Gαolf activates adenylyl cyclase III, raising cAMP and opening cyclic-nucleotide-gated cation channels; the entering Ca²⁺ then opens Ca²⁺-activated Cl⁻ channels, and Cl⁻ efflux amplifies the depolarisation. Neurons expressing the same receptor converge on one or two glomeruli, creating an odour map. Axons form fila olfactoria, pass through the cribriform plate, and synapse in glomeruli of the olfactory bulb. Mitral and tufted cells project via the lateral olfactory tract to the piriform cortex, amygdala and hippocampus, linking smell with emotion and memory. Adaptation occurs when Ca2+ entering through CNG channels activates calmodulin, which inhibits further channel opening and stimulates phosphodiesterase to lower cAMP, reducing sensitivity.

(d)(i) Gastrulation in frog

In the frog blastula, gastrulation begins on the dorsal side opposite the point of sperm entry, where the grey crescent lies. Bottle cells constrict and form a slit-like dorsal lip of the blastopore, which then extends laterally and ventrally into a circular blastopore. Involution begins as prospective endoderm and mesoderm roll over the lip and move inward, forming the archenteron. At the same time the animal-cap ectoderm spreads over the whole embryo by epiboly, and the involuted mesoderm elongates along the axis by convergent extension. The process converts the blastula into a three-layered gastrula. The dorsal lip is the Spemann-Mangold organizer; when transplanted it can induce a secondary axis, showing organizer activity. Cells migrating through the blastopore include presumptive notochord, somites, lateral mesoderm and endoderm. As the blastopore closes, the archenteron becomes the gut, and the body axes are established. During closure the exposed yolk-rich endodermal cells form the yolk plug, which becomes internalised as the blastopore constricts.

(d)(ii) Biogenetic law

Haeckel’s biogenetic law stated that ontogeny recapitulates phylogeny: embryonic development briefly repeats ancestral adult forms. Its evolutionary significance was to show that embryos of related vertebrates share common features, such as pharyngeal pouches and tails, supporting descent with modification. In vertebrate embryos, early stages of fish, amphibian, reptile, bird and mammal show similar pharyngeal arches. Modern embryology rejects strict recapitulation because development is not a simple replay of adult stages. Heterochrony, changes in timing and rate of developmental events, and paedomorphosis, retention of juvenile traits in adults, explain similarities and differences. Von Baer's laws give the accepted formulation: general characters appear before special ones, and the embryo of a higher animal resembles the embryo, not the adult, of a lower animal. Thus the biogenetic law is a historical insight, not a literal rule.

(e)(i) Stem cells

Stem cells are cells that self-renew and give rise to differentiated progeny. Potency defines their capacity: totipotent cells form all embryonic and extra-embryonic tissues; pluripotent cells form all three germ layers; multipotent cells form limited lineages; unipotent cells produce one cell type. Sources include embryonic stem cells, adult tissue stem cells such as haematopoietic and neural stem cells, and induced pluripotent stem (iPS) cells reprogrammed from somatic cells by the Yamanaka factors Oct4, Sox2, Klf4 and c-Myc (Nobel Prize 2012). Stem cells are maintained in specialised microenvironments called niches, and divide asymmetrically to give one stem cell and one committed progenitor. Their potency makes them central to developmental biology. They are important in regenerative medicine for tissue repair, disease modelling and potential cell therapies.

(e)(ii) Types of placenta

Mammalian placentas are classified by the histological barrier between maternal blood and fetal trophoblast. In epitheliochorial placentas, maternal epithelium remains, as in pig and horse; in syndesmochorial placentas, maternal connective tissue remains, a category classically given to ruminants such as sheep and cow, which are now regarded as synepitheliochorial; in endotheliochorial placentas, maternal endothelium remains, as in cat and dog; in hemochorial placentas, maternal blood directly contacts trophoblast, as in rodents and human. The barrier type affects exchange of gases, nutrients and waste. A thinner barrier generally increases exchange efficiency, while a thicker barrier offers more maternal-fetal separation. By the distribution of villi, placentas are diffuse (pig, horse), cotyledonary (cow, sheep), zonary (cat, dog) or discoidal (rodents, primates), and they are non-deciduate when no maternal tissue is shed at birth (pig, cow) or deciduate when it is (dog, human). Human placenta is hemochorial and supports efficient nutrient, gas and waste exchange, with IgG transfer by receptor-mediated FcRn rather than simple diffusion.