During sensitization in an allergic response, which sequence occurs?

• A. B lymphocytes produce antibodies immediately, leading to immediate symptoms.
• B. T helper cells become TH2 and signal B lymphocytes to produce antibodies that attach to mast cells.
• C. Mast cells release histamines directly in the first exposure.
• D. Cytotoxic T cells kill allergen-presenting cells.

Answer: (B)

During sensitization, helper T cells polarize to a TH2 profile and drive B lymphocytes to switch to producing IgE antibodies. These allergen-specific IgE molecules then bind to high-affinity receptors on mast cells and basophils, flagging them for later response. This sets up the sensitized state: upon re-exposure, the allergen cross-links the bound IgE, triggering mast cell degranulation and release of mediators like histamine that cause symptoms.

So the sequence described—TH2 cells signaling B cells to produce antibodies that attach to mast cells—fits the sensitization stage precisely. The other ideas describe events that don’t occur during sensitization: immediate symptoms from first exposure, direct histamine release without prior sensitization, or involvement of cytotoxic T cells, which isn’t the hallmark of this process.