Final answer:
B-1 cells are characterized by the surface expression of CD5 and IgM, which serve as the B-cell receptor on naive B cells and play a key role in the primary immune response. Option A.
Step-by-step explanation:
B-1 cells, a subset of B cells, are characterized on their surface by the expression of CD5 and a specific type of immunoglobulin. These cells primarily express IgM (Immunoglobulin M) on their surface, along with CD5.
IgM is the first antibody isotype expressed during the B cell development and is also found as the first antibody to be made in the blood plasma during primary immune responses. Unlike IgD, which is also found on the surface of B cells but is not secreted in significant quantities, IgM can be secreted by B cells.
When secreted, IgM antibodies are assembled into pentamers, making them very effective in binding antigens due to their multivalency. Furthermore, IgM's involvement is crucial for the primary immune response before other antibody isotypes, such as IgG, are produced in high quantities upon re-exposure to the same pathogen. So Option A.