The loose-fitting sac around the heart is the:

Monocytes are a type of agranulocytes, which are white blood cells that lack visible granules in their cytoplasm.
Agranulocytes also include lymphocytes, which are involved in adaptive immunity.
Choice A is wrong because basophils are a type of granulocytes, which are white blood cells that have granules in their cytoplasm.
Granulocytes also include neutrophils and eosinophils, which are involved in innate immunity.
Choice C is wrong because neutrophils are also a type of granulocyte.
Neutrophils are the most abundant white blood cells and are responsible for phagocytizing bacteria and fungi.
Choice D is wrong because eosinophils are also a type of granulocytes. Eosinophils are involved in allergic reactions and parasitic infections.
Normal ranges for white blood cells vary depending on age, gender, and health status, but generally, they are between 4,000 and 11,000 cells per microliter of blood.

It explains that the primary immune response produces antibodies within five to ten days of exposure to antigens, while the secondary immune response produces antibodies within a day or two of subsequent exposure to the same antigen.
This is because the primary immune response involves the activation and differentiation of naive B cells into plasma cells and memory cells, while the secondary immune response involves the rapid activation of memory cells into plasma cells.
Choice A is wrong because the primary and secondary immune responses can occur in both children and adults.
Choice B is wrong because the primary immune response is slower than the secondary immune response in producing antibodies.
Choice D is wrong because both primary and secondary immune responses are important for the body’s ability to protect itself from pathogens.
The primary immune response generates immunological memory, while the secondary immune response provides a faster and stronger response to repeated infections.