The ________ stores excess glucose and releases it into the blood when needed.

A. Complement system globulins: These are a group of plasma proteins synthesized primarily by the liver that circulate in an inactive state. They are activated by a cascade mechanism to enhance phagocytosis, inflammation, and cytolysis of pathogens. They are not specifically secreted by virus-infected cells to alert neighboring cells.

B. Interferons: When a cell is compromised by a viral pathogen, it synthesizes and releases these signaling proteins as a paracrine defense. Interferons bind to receptors on adjacent healthy cells, triggering the production of antiviral proteins that inhibit viral replication. They act as a critical early warning system in the innate immune response.

C. Granzymes: These are proteolytic enzymes released by natural killer cells and cytotoxic T lymphocytes via exocytosis. They enter a target cell through pores to induce programmed cell death or apoptosis. Their function is the direct destruction of infected or neoplastic cells rather than the signaling of neighbors.

D. Pyrogens: These substances, such as interleukin 1, act on the hypothalamus to elevate the body's temperature set point, resulting in fever. While they are part of the systemic inflammatory response, they do not provide localized cellular protection against viral entry. They modulate systemic thermoregulation rather than cellular viral resistance.

E. Perforins: These are pore-forming proteins secreted by effector lymphocytes to compromise the structural integrity of a target cell's plasma membrane. By creating holes in the membrane, they allow granzymes to enter and facilitate rapid cytolysis. They are tools of direct cellular execution rather than prophylactic intercellular signaling.

A. 6: Hemoglobin lacks the structural capacity to bind 6 oxygen molecules simultaneously. The protein is a tetramer, meaning it consists of only 4 polypeptide subunits. Each subunit is limited to the binding of a single heme group and its associated iron ion.

B. 2: This number represents only half of the potential carrying capacity of a fully saturated hemoglobin molecule. In the high partial pressure of oxygen found in pulmonary capillaries, hemoglobin typically binds more than two molecules. This would reflect a low oxygen saturation level of 50%.

C. 3: Binding 3 molecules would result in 75% oxygen saturation, which occurs as blood unloads oxygen to resting tissues. However, it does not represent the maximum theoretical or physiological limit of the transport protein. The molecular structure allows for one additional binding site to be filled.

D. 4: Each hemoglobin molecule is a tetramer composed of 4 globin chains, each containing a central heme group with a ferrous iron atom. Each iron atom can reversibly bind 1 molecule of O2. Therefore, a single hemoglobin molecule can carry a maximum of 4 oxygen molecules.

E. 5: Human hemoglobin does not possess a fifth binding site or heme group to accommodate an extra oxygen molecule. The quaternary structure is strictly limited to 4 subunits. Any value above 4 is biologically and chemically impossible for a standard adult hemoglobin A molecule.