(a) Define immunity. Explain cell-mediated immune response in vertebrates with suitable diagram. (b) Describe the structure and…

(a) Immunity and cell-mediated immune response

Immunity is the capacity of the body to resist pathogens, toxins and foreign or abnormal cells through innate (non-specific) and adaptive (specific, with memory) defences.

Cell-mediated immunity is brought about by T lymphocytes, not antibodies, and acts against intracellular pathogens, tumour cells and grafts. The labelled diagram should show:

• a dendritic cell or macrophage presenting antigen peptide on MHC-II to the T-cell receptor of a CD4⁺ helper T cell, with CD28–B7 co-stimulation
• the activated helper cell secreting IL-2 and interferon-γ
• interferon-γ activating macrophages to kill ingested microbes
• IL-2 expanding CD8⁺ cytotoxic T cells, which recognise peptide on MHC-I of an infected or tumour cell
• perforin and granzymes (or Fas–FasL) driving the target cell into apoptosis
• memory T cells remaining after the response, and regulatory T cells switching it off

(b) Structure and functions of mammalian placenta

The placenta is a temporary feto-maternal organ formed from the chorion (trophoblast with extra-embryonic mesoderm) and the uterine decidua basalis. Its form varies: diffuse in the pig, cotyledonary in ruminants, zonary in carnivores and discoid in primates.

Structure (human, discoid, haemochorial): at term it weighs about 500 g, is 15–20 cm wide and 2–2.5 cm thick, with 15–20 cotyledons on the maternal face and the umbilical cord (two arteries, one vein) on the fetal face. Branching chorionic villi containing fetal capillaries dip into intervillous spaces filled with maternal blood from spiral arteries. The placental barrier consists of syncytiotrophoblast, cytotrophoblast, basement membrane, villous connective tissue and capillary endothelium.

Functions:

• exchange of O₂, CO₂, nutrients and urea
• endocrine: hCG (maintains the corpus luteum), progesterone, oestrogens, human placental lactogen
• transfer of maternal IgG; barrier to most microbes
• immune tolerance of the fetus

(c) Functions of neutrophils, basophils and lymphocytes

Neutrophils (40–70% of leucocytes; multilobed nucleus) are the first cellular defenders against bacteria and fungi. They leave capillaries by diapedesis, follow chemotactic signals, phagocytose microbes and kill them with lysosomal enzymes and the respiratory burst of reactive oxygen species. They also release neutrophil extracellular traps, and dead neutrophils form pus.

Basophils (under 1%) carry IgE on FcεRI receptors. On allergen contact they degranulate, releasing histamine, heparin and leukotrienes, which cause vasodilation, increased permeability and allergic or anaphylactic reactions. They also secrete IL-4 and IL-13, which favour Th2 responses and help against helminths and ticks.

Lymphocytes (20–40%) are the cells of specific immunity:

• B cells become plasma cells that secrete antibodies; some become memory cells
• helper T cells secrete cytokines that coordinate immunity; cytotoxic T cells kill infected and tumour cells; regulatory T cells suppress responses
• natural killer cells kill virus-infected and tumour cells without prior sensitisation

(d) Counter-current mechanism of the vasa recta and its importance

The vasa recta are long hairpin-shaped capillaries of juxtamedullary nephrons that run parallel to the loop of Henle into the medulla. They are a counter-current exchanger, which preserves the medullary gradient made by the loop of Henle (the counter-current multiplier). The labelled diagram should show:

• the descending limb carrying blood into the medulla, where interstitial osmolarity rises from about 300 mOsm/L (cortex) to about 1200 mOsm/L (inner medulla)
• water leaving, and NaCl and urea entering, the descending limb
• the ascending limb, where water re-enters and NaCl and urea diffuse out
• blood leaving the medulla only slightly hyperosmotic to plasma

Importance: the exchange and slow flow stop solutes being washed away. The preserved gradient lets the ADH-controlled collecting duct reabsorb water osmotically and produce concentrated urine, conserving body water. The vasa recta also supply the medulla with oxygen and remove the reabsorbed water.

(e) Biogenetic law and Haeckel's theory of recapitulation

The biogenetic law, proposed by Ernst Haeckel in 1866 (Generelle Morphologie der Organismen), states that "ontogeny recapitulates phylogeny": the embryonic development of an individual briefly repeats the evolutionary history of its race.

Characteristics of Haeckel's theory:

• ontogeny is a short and rapid repetition of phylogeny
• the embryo passes through stages resembling adult ancestors, for example the pharyngeal (gill) slits of all vertebrate embryos
• terminal addition: new characters are added at the end of development, pushing ancestral adult stages back into the embryo
• condensation: ancestral stages are telescoped into a short embryonic period
• embryology is therefore evidence of common descent

Limitations: von Baer (1828) showed that embryos diverge from general to special features and never pass through adult forms of other animals; Haeckel's drawings exaggerated embryonic similarity. Only a modified idea survives: early embryos share conserved features, such as pharyngeal arches, because of common ancestry.