Megakaryocytes are responsible for producing platelets, the remnants of cells that help form clots.

B-cells and T-cells are responsible for humoral and cell-mediated immunity. Macrophages ingest bacteria and viruses and present them to B- and T-cells to initiate an immune reaction.

Plasma cells are differentiated B-cells that serve to secrete antibodies after being stimulated by a helper T-cell.

Natural killer cells are also stimulated by helper T-cells and secrete perforin to kill invading pathogens. Memory B-cells are differentiated B-cells that are specialized to detect a re-infection by the same pathogen, allowing for a quick immune response. Macrophages ingest infecting agents.

Endosomes function to shuttle phagocytosed material to the lysosome, where cellular digestion can take place. This means that professional phagocytes, such as macrophages, can be expected to have a larger number of endosomes than other cells that are less specialized for this process.

The role of a plasma cell in the immune system is to produce antibodies. These antibodies are effective in binding to extracellular pathogens. Antibodies produced by plasma cells would not be effective in binding to intracellular pathogens, such as M. tuberculosis and intracellular HIV. Only one answer choice specifies an extracellular pathogen.

Plasma cells are developed from B-lymphocyte precursors in response to the presence of a specific antigen, and are part of the adaptive immune response. As such, they would be relatively ineffective at fighting a newly-evolved microbe to which the body has never been exposed before.

Both B-lymphocytes and T-lymphocytes are derived from lymphoid progenitor cells, a division of hematopoietic stem cells. These progenitors are housed in red bone marrow. B-lymphocytes remain in the red bone marrow to mature, but T-lymphocytes transition to the thymus for positive selection.

The lymph nodes and spleen are secondary immune tissues, responsible for housing mature B- and T-lymphocytes in order to carry out the immune response. The lymph nodes and spleen are not responsible for immune cell development. Yellow bone marrow is primarily used for fat storage and is not involved in the lymphatic system or immune response.

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.