Reproduction, Development and the Immune System
Cycles, layers, specificity
This unit links hormonal cycles, gametogenesis, embryonic germ layers and immune specificity. The exam likes tracing one hormone or one cell type through its entire pathway, so practise writing each pathway end to end.
Core rules
- Gametogenesis: spermatogenesis is continuous from puberty and yields four sperm per meiosis; oogenesis begins before birth, pauses in prophase I, and completes only at fertilisation, producing one ovum and polar bodies.
- Menstrual cycle: FSH drives follicle growth and oestrogen; an oestrogen surge triggers the LH surge and ovulation near day 14; the corpus luteum secretes progesterone, which maintains the endometrium; without implantation it degenerates and menstruation follows; embryonic hCG rescues the corpus luteum.
- Development: cleavage, blastula, gastrulation (ectoderm, mesoderm, endoderm), organogenesis; differentiation reflects differential gene expression, not DNA loss; induction and apoptosis shape organs; the placenta exchanges materials without mixing maternal and fetal blood.
- Immunity: innate defences (skin, mucus, phagocytes, inflammation) versus adaptive; B cells become plasma cells that secrete antibodies; helper T cells (CD4) activate B cells and cytotoxic T cells (CD8); MHC presents antigen; clonal selection expands matching clones; memory cells give a faster, larger secondary response; vaccines give active immunity, injected antibodies give passive immunity.
Worked example
On second exposure to the same pathogen, memory B and T cells recognise the antigen and proliferate within days rather than weeks; antibody titre rises higher and faster, and disease is prevented or mild. Reasoning: the clone already exists in large numbers, so the lag phase of the primary response is skipped.
Marks lost
Attributing antibody secretion to T cells; placing memory cells in innate immunity; saying the placenta mixes maternal and fetal blood; confusing the oestrogen and progesterone peaks.