**Core Concept**
Helper cells, also known as T helper cells, are a type of immune cell that plays a crucial role in the adaptive immune response. They assist in activating and directing other immune cells, such as B cells and cytotoxic T cells, to fight infections.

**Why the Correct Answer is Right**
Helper cells belong to the CD4+ T cell subset, which is characterized by the presence of the CD4 receptor on their surface. These cells recognize and bind to antigens presented by MHC class II molecules on antigen-presenting cells (APCs), such as dendritic cells and macrophages. Upon activation, helper cells secrete cytokines that promote the activation and proliferation of other immune cells, such as B cells and cytotoxic T cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Helper cells do not belong to the B cell lineage, as they are distinct from B cells in terms of their surface receptors and functional roles.
**Option B:** Helper cells are not a type of cytotoxic T cell, as they do not directly kill infected cells or tumor cells.
**Option C:** Helper cells are not a type of natural killer (NK) cell, as they do not recognize and kill target cells through a different set of receptors.

**Clinical Pearl / High-Yield Fact**
Helper cells play a critical role in the development of allergic responses and autoimmune diseases, such as multiple sclerosis and rheumatoid arthritis. Understanding the function of helper cells is essential for the development of immunosuppressive therapies for these conditions.

**Correct Answer: C. Helper T cells.