Which one of the following statements best describes the role of B cells and T cells in the human body?

The correct option is: They protect the body from the diseases caused by pathogens.

Detailed Explanation:
B cells and T cells are specialized types of white blood cells known as lymphocytes. They are the key cellular components of the adaptive (acquired) immune system, which provides highly specific and long-lasting defense mechanisms against foreign invaders.

1. Role of B Cells (Humoral Immunity):
B cells mature in the bone marrow and are responsible for antibody-mediated immunity. When a B cell encounters a specific antigen on a pathogen (such as a bacterium or virus), it gets activated and differentiates into a plasma cell. These plasma cells produce and release large quantities of proteins called antibodies (immunoglobulins). Antibodies circulate in the blood and lymph, binding specifically to the target pathogens to neutralize them or mark them for destruction by other immune cells.

2. Role of T Cells (Cell-Mediated Immunity):
T cells mature in the thymus gland and are responsible for cell-mediated immunity. Unlike B cells, they do not produce antibodies. Instead, they identify and eliminate pathogens that have already invaded host cells. Helper T cells coordinate the immune response by releasing chemical messengers (cytokines) that activate B cells and other immune cells. Cytotoxic T cells directly bind to and kill infected host cells (e.g., virus-infected cells) and cancer cells, preventing the infection from spreading.

Why the other options are incorrect:
- They protect the body from environmental allergens: Environmental allergens (like pollen or dust) are harmless substances. While the immune system can react to them (causing allergies), this hypersensitivity is a misdirected response rather than their primary evolutionary purpose of defense.
- They alleviate the body's pain and inflammation: Lymphocytes do not act as pain relievers or anti-inflammatory agents. In fact, their activation during an infection often triggers inflammation to help recruit more immune cells to the affected site.
- They act as immunosuppressants in the body: Immunosuppressants act to reduce or prevent an immune response. Except for a specialized regulatory subset that helps maintain self-tolerance, the primary collective role of B and T cells is to active and drive the immune response to eliminate threats.