Before we look at the organs of the lymphoid system, let’s first review the principal cells of the lymphoid system, the lymphocytes. Lymphocytes are produced in primary lymphoid organs (bone marrow and thymus), and with the help of antigen presenting cells, such as macrophages and dendritic cells, they are activated to respond to specific antigens in secondary lymphoid organs (lymphatic nodules, lymph nodes, and spleen). There are two main types of lymphocytes, B and T lymphocytes.

• B lymphocytes or simply B cells, possess B cell receptors (BCRs) on their cell surfaces that bind antigens. In response to antigen binding, B cells proliferate and differentiate to give rise to plasma cells that produce a single type of antibody that is specific for that antigen. During this differentiation process, some of these activated B cells become memory B cells. These long-lived cells are capable of undergoing rapid activation to produce plasma cells when the body encounters that particular antigen again.
• T lymphocytes express T cell receptors (TCRs) on their cell surfaces and are found as several different subtypes in the body, including Helper T cells (CD4+), Cytotoxic T cells (CD8+), and Regulatory T cells. Helper T cells assist in activation of other lymphocytes and also form populations of memory cells that can be reactivated in response to future antigen exposure. Cytotoxic T cells are specialized for cell-mediated killing of virus-infected and foreign cells. The activation of cytotoxic T cells also yields populations of memory T cells. Finally, Regulatory T cells maintain tolerance of self-antigens and help keep immune responses in check.
• Now examine this blood smear slide, focusing your attention on the structure of the lymphocytes. Lymphocytes range in size of 6-15 microns in diameter and are sometimes described as small, medium, and large lymphocytes (see image below). They have a large, round centrally located nucleus with a thin rim of pale blue cytoplasm. It is not possible to distinguish the different types of lymphocytes (e.g. B vs. T cells) in a histological specimen without the help of immunocytology that allows for visualization of these populations based on the detection of specific cell surface proteins.
   

Next let's look at the thymus.