Final answer:
A toxoid being "cross-reactive" with a toxin means it can produce an immune response against various related toxins, helping prevent diseases by using inactivated bacterial toxins in vaccines.
Step-by-step explanation:
When we say that a toxoid is "cross-reactive" with a toxin, it refers to the ability of the toxoid to produce an immune response against a range of related toxins, which is option B. Toxoids are inactivated bacterial toxins used in vaccines to prevent diseases where bacterial toxins are significant in their pathogenesis. Cross-reactivity occurs when antibodies elicited by one antigen, like a toxoid, also recognize and bind to similar epitopes on different but related toxins.
For example, antibody cross-reactivity may occur against the surface structures of various Gram-negative bacteria, thereby providing immunity against multiple pathogens even if the immune system was exposed to or vaccinated against only one. However, this cross-reactivity can also be a double-edged sword; it can lead to autoimmune disorders if antibodies raised against pathogens mistakenly react with the body's own cells, as seen with antibodies that react with one's own DNA in systemic lupus erythematosus (SLE).