## **Core Concept**
Humoral immunity is a type of immunity that is mediated by secreted antibodies produced by **B cells**. It plays a crucial role in defending against pathogens, especially those found in the blood and lymphatic system. This form of immunity is essential for neutralizing extracellular pathogens.

## **Why the Correct Answer is Right**
The correct answer involves **B cells**, which are the cells responsible for producing and secreting antibodies. When a B cell encounters its specific antigen, it can become activated, proliferate, and differentiate into a **plasma cell**, which is the antibody-producing factory. These antibodies can then bind to specific pathogens, marking them for destruction or directly neutralizing them. This mechanism is fundamental to humoral immunity.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **T cells** are crucial for cell-mediated immunity and can assist in activating B cells, they are not directly involved in producing antibodies.
- **Option B:** **Macrophages** are involved in phagocytosis and play a role in innate immunity and antigen presentation, not directly in humoral immunity.
- **Option C:** This option might seem related but is not specified; however, based on common immune cells, **neutrophils** are primarily involved in innate immunity and are not the key players in humoral immunity.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that individuals with **B cell deficiencies** are particularly susceptible to recurrent infections, highlighting the importance of humoral immunity. A classic example is **Bruton's agammaglobulinemia**, a condition characterized by the lack of mature B cells and, consequently, very low levels of antibodies.

## **Correct Answer:** B. B cells.