Write your answer in about 150 words for each of the following : 10×5=50 (a) Define immunity. Explain cell-mediated immune…

(a) Immunity and Cell-Mediated Immune Response

Immunity is the physiological state of an organism characterized by resistance against pathogenic microorganisms, foreign antigens, or abnormal cells. It comprises non-specific innate defenses and highly specific adaptive immunity, which is subdivided into humoral (antibody-mediated) and cell-mediated branches.

Cell-mediated immune response (CMIR) is orchestrated by T-lymphocytes without the involvement of free circulating antibodies. Antigen-presenting cells (APCs), such as dendritic cells and macrophages, engulf intracellular pathogens, process them, and display peptide fragments bound to Major Histocompatibility Complex (MHC) molecules on their surface.

`` [ Pathogen/Infected Cell ] │ ▼ [ APC processes ] ──(MHC-II:Antigen)──► [ CD4+ Helper T-cell ] [ Antigen ] │ │ (IL-2 / Cytokines) (MHC-I:Antigen) │ │ ▼ └─────────────────────────────► [ CD8+ Cytotoxic T-cell ] │ (Perforin / Granzymes) │ ▼ [ Target Cell Lysis/Apoptosis ] ``

Naïve CD4+ helper T-cells (Th) recognize antigen-MHC Class II complexes via their T-cell receptors (TCR) alongside CD28-B7 co-stimulation, subsequently secreting cytokines like Interleukin-2 (IL-2) and Interferon-gamma (IFN-γ). These cytokines activate CD8+ cytotoxic T-lymphocytes (Tc) and macrophages. Activated Tc cells recognize specific antigens presented by MHC Class I molecules on virus-infected or transformed cells, inducing apoptosis via the secretion of perforins, granzymes, or Fas-Fas ligand interactions. Regulatory T-cells (Treg) concurrently modulate the response to prevent autoimmune pathology.

(b) Structure and Functions of Mammalian Placenta

The mammalian placenta is a composite, temporary feto-maternal organ that develops during gestation to sustain the fetus. Structurally, it consists of a fetal component derived from the chorion (trophoblast layered with extraembryonic mesoderm) and a maternal component formed by the modified uterine endometrium (decidua basalis). Finger-like projections, termed chorionic villi, branch extensively and project into maternal blood sinuses. In the human hemochorial placenta, maternal blood enters the intervillous spaces via maternal spiral arteries, directly bathing the chorionic villi. The placental barrier consists of syncytiotrophoblast, cytotrophoblast, fetal connective tissue, and fetal capillary endothelium, preventing direct mixing of fetal and maternal blood while enabling selective exchange.

`` Maternal Blood Pool (Decidua Basalis) ◄──► [ Placental Barrier ] ◄──► Fetal Capillaries (Chorionic Villi) ``

Functionally, the placenta mediates physiological exchange by facilitating the diffusion of oxygen, glucose, amino acids, and fatty acids to the fetus, and clearing carbon dioxide and urea into maternal circulation. It acts as an active endocrine organ, synthesizing human chorionic gonadotropin (hCG) to maintain the corpus luteum, along with progesterone, estrogens, and human placental lactogen (hPL) to support gestation and mammary gland development. Furthermore, it serves as an immunological barrier by conferring passive immunity via the transcytosis of maternal IgG while suppressing maternal anti-fetal immune responses through non-classical HLA-G expression.

(c) Functions of Neutrophils, Basophils, and Lymphocytes

Neutrophils, basophils, and lymphocytes represent distinct leukocyte lineages that execute innate and adaptive immune defense:

Neutrophils are polymorphonuclear phagocytes that constitute the first cellular line of innate defense. They migrate rapidly to infection sites via chemotaxis, engulfing opsonized bacteria into phagosomes that fuse with lysosomes. Microbial destruction occurs via the respiratory burst, generating reactive oxygen species (ROS) such as superoxide and hypochlorous acid. Neutrophils also release Neutrophil Extracellular Traps (NETs)—web-like chromatin scaffolds studded with antimicrobial peptides—to ensnare and neutralize extracellular pathogens.

Basophils are circulating granulocytes expressing high-affinity IgE receptors (FcεRI). Upon antigen-mediated IgE cross-linking, they degranulate, releasing vasoactive mediators including histamine, heparin, leukotrienes, and platelet-activating factor. These substances induce vasodilation and increase vascular permeability, mediating allergic responses and antiparasitic immunity against helminths.

Lymphocytes direct adaptive and innate surveillance: B-lymphocytes differentiate into antibody-secreting plasma cells, producing immunoglobulins (IgM, IgG, IgA, IgE, IgD) to neutralize antigens and fix complement, while retaining memory B-cells. T-lymphocytes mediate cellular immunity; Th cells coordinate immune cascades via cytokines, while Tc cells directly lyse abnormal and virally infected cells. Natural Killer (NK) cells provide innate cytotoxicity against cells displaying downregulated MHC-I molecules or stress-induced ligands.

(d) Counter-Current Mechanism by Vasa Recta and Physiological Importance

The counter-current mechanism in the vasa recta operates as a counter-current exchanger to preserve the hyperosmotic medullary gradient established by the loops of Henle. The vasa recta are hairpin-shaped peritubular capillaries descending deep into the renal medulla parallel to the loops of Henle.

`` [ Cortex: 300 mOsm/kg ] Descending Limb Ascending Limb (Blood Inflow) (Blood Outflow) │ ▲ │ H2O exits │ H2O enters │ Solutes enter │ Solutes exit ▼ │ [ Deep Medulla: 1200 mOsm/kg ] ``

As blood flows down the descending limb into the hypertonic medullary interstitium, water diffuses out of the capillary into the interstitium, while solutes (NaCl and urea) diffuse into the blood. As blood turns the hairpin loop and flows up the ascending limb toward the cortex, the surrounding interstitium becomes progressively less concentrated; consequently, solutes diffuse out of the capillary back into the interstitium, while water is drawn back into the capillary.

Physiologically, this passive exchange prevents blood flow from washing away the hyperosmotic medullary solute gradient (increasing from 300 mOsm/kg at the corticomedullary junction to 1200 mOsm/kg at the papillary tip). Maintaining this hyperosmolar interstitium enables the collecting ducts, under the regulation of antidiuretic hormone (ADH), to reabsorb water osmotically, thereby producing hypertonic urine and conserving body water.

(e) Biogenetic Law and Characteristics of Haeckel's Theory of Recapitulation

The Biogenetic Law, formulated by Ernst Haeckel in 1866, is summarized by the aphorism "ontogeny recapitulates phylogeny." It posits that the developmental stages of an individual organism (ontogeny) represent a condensed, sequential repetition of the evolutionary adult stages of its ancestors (phylogeny).

Haeckel’s theory of recapitulation is characterized by two fundamental premises: the principle of terminal addition, which assumes that evolutionary novelties are added exclusively to the final adult stage of ancestral ontogeny, and the principle of condensation, which posits that ancestral developmental pathways are accelerated in time to accommodate new features within embryonic life. For example, Haeckel interpreted the transient presence of pharyngeal pouches and a post-anal tail in mammalian embryos as a direct replay of adult ancestral fish stages.

The theory faced severe scientific refutation. Karl Ernst von Baer's laws of embryology demonstrated that embryos share generalized taxonomic characteristics early in ontogeny, diverging into specialized traits later, without passing through adult forms of lower taxa. Modern evolutionary-developmental biology (evo-devo) rejects strict recapitulation, proving that evolution proceeds by modifying developmental timing (heterochrony), gene regulatory networks, and embryonic structures directly, rather than by appending traits onto fixed ancestral adult templates.