Final answer:
The Brambell receptor and FcRB (also known as FcRn) are responsible for transporting IgG from blood into extracellular spaces; thus the correct answer is both (d) a and b.
Step-by-step explanation:
The receptors that help transport IgG from blood into extracellular spaces are known as Brambell receptors, also referred to as Fc receptors, specifically FcRB or the neonatal Fc receptor (FcRn). These receptors bind to IgG antibodies at the acidic pH of the endosome following pinocytosis and facilitate the transport of IgG through cells and into tissue spaces. This process helps maintain IgG serum levels, provides passive immunity to the fetus during pregnancy, and can deliver IgG-based therapies across biological barriers.
The correct answers to the question 'Which of the following receptors helps transport IgG from blood into extracellular spaces?' are both a. Brambell Receptor and b. FcRB, which is designated by d. A & B in the given options.