Final answer:
B. They show a very high degree of variability. Pattern recognition receptors (PRRs) do not have a very high degree of variability; they are rather limited by genetic encoding and membrane space constraints. They are encoded in the germline and are able to recognize pathogen classes while being expressed by various cell types of the innate immune system.
Step-by-step explanation:
Pattern recognition receptors (PRRs) are not described by showing a very high degree of variability. PRRs are essential components of the innate immune system, recognizing pathogen-associated molecular patterns (PAMPs) on the surface of pathogens. These molecular signatures, which can be carbohydrate, polypeptide, or nucleic acid structures, are distinct from host cell molecules. However, in contrast to the adaptive immune system which features a large diversity of highly specific receptors, PRRs are limited in variety due to two main constraints: the need for each receptor type to be encoded by a specific gene and the finite surface area available on the cell membrane for receptor expression.
PRRs are indeed encoded in the germline and expressed by various cell types including phagocytes, NK cells, macrophages, and dendritic cells. They are known to recognize classes of pathogens, but their variability is not as extensive as that of the adaptive immune system's receptors, which are generated through recombination and can recognize a plethora of unique antigens.