Write on the following in about 150 words each: (a) Functions of immunoglobulins (10 marks) (b) ABO system of blood groups and…

(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 is based on carbohydrate antigens on red-cell membranes. 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 prophylactically prevents maternal sensitization.

(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 receptors on cilia; Gs activates adenylyl cyclase, raising cAMP, opening cyclic-nucleotide-gated channels, depolarizing the neuron. 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, the dorsal lip of the blastopore forms first. Involution begins as prospective endoderm and mesoderm roll over the lip and move inward, forming the archenteron. The process converts a two-layered 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, neural tube and other mesodermal tissues. As the blastopore closes, the archenteron becomes the gut, and the body axes are established.

(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. Thus the 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 cells reprogrammed from somatic cells. 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 cow; in syndesmochorial placentas, maternal connective tissue remains; in endotheliochorial placentas, maternal endothelium remains, as in horse; in hemochorial placentas, maternal blood directly contacts trophoblast, as in cat 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. Human placenta is hemochorial and supports efficient nutrient, gas and waste exchange, with IgG transfer by receptor-mediated FcRn rather than simple diffusion.

Thus, immunoglobulins, blood-group compatibility, olfactory signalling, gastrulation, stem-cell potency and placental barriers together describe the molecular, developmental and physiological mechanisms that maintain immunity, reproduction and organismal form.