Theory: Human Physiology / Immunology
1. Neonatal Immunity
Newborns have immature immune systems. Two forms of passive immunity protect them: Transplacental IgG: provides systemic immunity for first 3-6 months. Colostrum/breast milk IgA: protects gut and respiratory mucosa. Benefits of breastfeeding: reduced risk of gastrointestinal infections, respiratory infections, allergies, sudden infant death syndrome (SIDS), obesity. WHO recommends exclusive breastfeeding for 6 months.
2. Secretory IgA Structure
Secretory IgA differs from serum IgA: Serum IgA = monomer (2 H+2 L chains). Secretory IgA = dimer (2 monomers joined by J chain + secretory component). Secretory component: added by mucosal epithelial cells during transcytosis. Protects IgA from proteolytic degradation in gut. Allows IgA to function in the hostile environment of mucosal surfaces (saliva, tears, gut, respiratory tract, breast milk).
3. Immunoglobulin Classes and Their Properties
IgM (900 kDa, pentamer): first antibody in primary immune response; excellent complement activator; ABO agglutinins. IgG (150 kDa, monomer): most abundant serum antibody; crosses placenta; long-lived; opsonises bacteria. IgA (160-320 kDa, monomer/dimer): mucosal immunity; dominant in secretions. IgE (190 kDa, monomer): binds mast cells; type I hypersensitivity (allergy); parasite defence. IgD: B cell surface receptor; role in B cell activation.
4. Vaccination and Passive Immunity
Vaccines stimulate ACTIVE immunity (long-lasting, immunological memory). Passive immunity examples: Anti-tetanus serum (ATS/TIG): ready-made anti-tetanus antibodies for immediate protection. Anti-venom: snake/scorpion bite treatment with antibodies. Intravenous immunoglobulin (IVIG): pooled IgG from donors for immunodeficient patients. Monoclonal antibodies: targeted therapy (trastuzumab for HER2+ breast cancer). Colostrum: natural passive immunity for neonates.
Frequently Asked Questions
1. What is colostrum? ⌄
Colostrum is the first milk produced by mammary glands (Days 1-3-5 post-delivery). It is: yellowish, thick, and highly concentrated in immune factors. Rich in: secretory IgA (sIgA), IgG, IgM, lactoferrin, lysozyme, leukocytes, cytokines, growth factors, proteins. Lower in fat and carbohydrates than mature milk. It provides critical passive immunity and growth factors to the neonate.
2. Why is IgA important in colostrum? ⌄
Secretory IgA (sIgA) in colostrum: (1) Resists digestion by proteases in the neonatal gut (protected by secretory component). (2) Coats the intestinal lining of the newborn, preventing bacteria and viruses from adhering to and penetrating the gut epithelium. (3) Provides immune exclusion — keeps pathogens out of the body without causing inflammation. (4) This mucosal immunity is especially critical in the first days when neonatal gut epithelium is immature.
3. What is passive immunity from mother to baby? ⌄
Passive immunity: transfer of ready-made antibodies (not generated by baby's own immune system). Two routes: Placental transfer: IgG is the ONLY antibody class that crosses the placenta (via FcRn receptors). Provides immunity from 3rd trimester. Protects infant for first few months. Colostrum/breastfeeding: IgA and some IgG in breast milk → passive protection of gut mucosa. Passive immunity is temporary (antibodies degraded over time), not long-lasting like active immunity.
4. Which immunoglobulin crosses the placenta? ⌄
IgG is the ONLY immunoglobulin class that crosses the placenta in humans (via FcRn neonatal Fc receptors). IgA, IgM, IgD, IgE do NOT cross the placenta. IgG provides primary systemic passive immunity to the neonate from birth until the infant's own immune system matures (around 3-6 months). This explains why premature infants have lower IgG levels and are more susceptible to infections.
5. What are the 5 classes of immunoglobulins? ⌄
IgG: most abundant (75%), crosses placenta, long-lived, secondary immune response. IgA: mucosal immunity, secretory IgA in breast milk, saliva, tears. IgM: first antibody in primary response, pentameric, largest, ABO blood group agglutinins. IgE: allergies and parasitic defence, binds mast cells/basophils. IgD: B cell surface receptor, function not fully understood.