It's tricky to pick just three main cells in the immune system, as they all work together in a complex network. However, we can highlight three essential cell types that play crucial roles in different aspects of immunity:

1. Lymphocytes: These are the "soldiers" of the immune system, specializing in recognizing and attacking specific invaders. There are two major types:

* T cells: Directly attack infected cells or regulate other immune cells.

* B cells: Produce antibodies that bind to pathogens and neutralize them.

2. Phagocytes: These are the "clean-up crew" of the immune system, engulfing and destroying pathogens and cellular debris. Examples include:

* Macrophages: Large, long-lived phagocytes that can also present antigens to T cells.

* Neutrophils: Abundant and fast-acting phagocytes, particularly effective against bacteria.

3. Antigen-Presenting Cells (APCs): These cells "show" the invaders (antigens) to the lymphocytes so they can learn to target them. Major APCs include:

* Dendritic cells: Specialized APCs that are particularly good at activating naive T cells.

* Macrophages: Can also act as APCs.

While these three cell types are essential, many other cells like mast cells, natural killer cells, and various cytokine-producing cells also contribute to immune function.