T-lymphocytes and B-lymphocytes both arise in the bone marrow from the lymphoid progenitor lineage of stem cells. B-lymphocytes remain in the bone marrow for maturation, while T-lymphocytes migrate to the thymus. In the thymus, T-lymphocytes are exposed to antigens from the body's own cells. If the T-cell reacts to the antigen, it is destroyed to prevent autoimmune disorders. This process is known as positive selection.

The thyroid is an endocrine gland, and is not a site for immune cell development. The lymph nodes are secondary immune tissues and are responsible for conducting the immune response and housing mature lymphocytes, not for immune cell development. 

Cytotoxic T-lymphocytes, also known as kill T-cells, directly destroy infected cells by releasing their cytotoxic granules into the cells and causing them to lyse.

Helper T-lymphocytes produce cytokines, which are involved in cell signaling and propagating the immune response. Follicular helper T-lymphocytes control B-cells in lymph nodes. Regulatory T-lymphocytes inhibit the immune response. 

The immune system can be broken down into two main categories: innate and adaptive. Innate immunity includes nonspecific defense mechanisms, and the adaptive side is broken down into two primary sections, humoral and cell-mediated immunity. The key players in cell-mediated immunity are T-cells (including helper, suppressor, memory and cytotoxic T-cells). The humoral response occurs through B-cells, which are the precursors for plasma cells and memory cells. Once a B-cell is exposed to a matching antigen, it will begin to produce two types of daughter cells: plasma cells and memory cells. Plasma cells produce large amounts of antibodies in order to fight the infection at hand, where memory cells will remain in the lymph nodes for the rest of the organism's life. Memory cells are key in an organism's quick secondary response to a microbe that was previously encountered.

Granulocytes are cells categorized because they have vesicles within their membrane that look similar to a granule. Basophils, eosinophils and neutrophils all present granule-like figures and are categorized as granulocytes. Although they derive from the same myeloid stem cells as the granulocytes, monocytes are categorized as agranulocytes.

In terms of MHC restriction, students should be familiar with the fact that cytotoxic T-cells are CD8+ and MHC I restricted. The alternative T subset, the helperT-ell, is CD4+ and MHC II restricted. Both cells rely on cytokines for growth, survival, and their effector functions. 

Eosinophils are unable to produce antibody, which are limited to the B cell lineage.  

Generally, cytotoxic T cells express CD8, express MHC I, are adept at killing specific targets, and while they can produce some cytokines, helper T cells predominantly secrete more different kinds of cytokines and larger quantities of cytokines. Helper T cells express CD4, MHC II, and function primarily in producing cytokines to activate and induce other immune cells to function in the inflammatory response.

The only choice that is not a function of any type of T-cell is the direct production of antibodies (which is performed by B-cells). Cytotoxic T-cells kill other cells that are bound to antigen/MHC-I complexes. Suppressor T-cells tone down the response of both B- and T-cells, and helper T-cells secrete cytokines, which increase the activity of many other immune cell types.

Innate defenses, such as the skin or macrophages, are the first line of defense against infection. Other responses only become effective if a pathogen cannot be repelled by innate mechanisms.

All of the following are examples of non-specific defense mechanisms of the immune system, except for antibody production by B-cells. B-cells respond to specific antigens within the body via immunoglobulins located on the plasma membrane of B-cells. This type of response is known as adaptive immunity, and develops only after a particular pathogen has invaded the immune system.