Final answer:
Antigens are recognized by T and B cells and can trigger an immune response. They are molecules identified as foreign and can consist of proteins, carbohydrates, or other substances. Activation of B cells by antigens differs based on their molecular structure, resulting in T-dependent or T-independent pathways.
Step-by-step explanation:
The true statement about antigens from the options provided is: C) They are recognized by T and B cells. Antigens are not always derived from pathogens nor are they always proteins, as they can also be carbohydrates or other substances. Furthermore, antigens do not usually cause cellular damage; rather, they trigger an immune response.
Antigens are molecules that B cells and T cells recognize as foreign, and they are found not only on pathogens like bacteria and viruses but also on cancer cells and transplanted organs. These molecules can trigger the immune system to react against the cells that carry them. Activation of B cells occurs through different mechanisms depending on the molecular class of the antigen. Protein antigens require the B cell to function as an antigen-presenting cell and present the protein epitopes with MHC II to helper T cells, categorized as T-dependent antigens. Nonprotein antigens (polysaccharides, lipopolysaccharides, etc.) are known as T-independent antigens because they can activate B cells without T cell help.
Thus, the ability of T and B cells to recognize specific antigens allows the immune system to mount a targeted defense against foreign molecules, initiating a cascade of events to eliminate the invading organisms or cells.