Humoral immunity is mediated by:

Options

A. plasma cell
B. eosinophil
C. neutrophil
D. Cytotoxic T-cell

Answer

**Step-by-Step Solution:**

1. **Understanding Humoral Immunity**: Humoral immunity is a component of the immune response that is mediated by antibodies. It primarily involves the production of antibodies by B cells (B lymphocytes) in response to antigens.

2. **Identifying the Key Players**: The main players in humoral immunity are:
- **B cells**: These are lymphocytes that, upon activation by an antigen, differentiate into plasma cells.
- **Plasma cells**: These are the effector form of B cells that produce and secrete antibodies.

3. **Examining the Options**:
- **A. Plasma cells**: Correct. Plasma cells are responsible for producing antibodies, which are crucial for humoral immunity.
- **B. Eosinophils**: Incorrect. Eosinophils are primarily involved in allergic responses and combating parasitic infections, not in humoral immunity.
- **C. Neutrophils**: Incorrect. Neutrophils are a type of white blood cell that primarily engage in phagocytosis, which is part of the innate immune response, not humoral immunity.
- **D. Cytotoxic T cells**: Incorrect. Cytotoxic T cells are involved in cell-mediated immunity and directly kill infected cells rather than producing antibodies.

4. **Conclusion**: The correct answer is **A. Plasma cells**, as they are the cells that mediate humoral immunity by producing antibodies that circulate in body fluids.