Immune cells communicate with one another through signaling molecules called cytokines. These proteins are released by various immune cells in response to infection or injury and travel to target cells by binding to specific receptors. The result is coordinated actions across the immune system—activating cells, promoting or limiting inflammation, guiding cell growth and differentiation, and helping recruit cells to sites of infection. Cytokines include many different types, such as interleukins, interferons, and tumor necrosis factors. Chemokines are a subset of cytokines whose main job is to direct cell movement toward areas of infection (chemotaxis), so they’re still part of the broader cytokine family. Antibodies, while crucial for recognizing and neutralizing pathogens, are not the signaling messages used for communication between immune cells. Hormones act as endocrine signals in the body and are not the primary means of immune-cell communication.

The body relies on the liver to make the majority of circulating plasma proteins. Hepatocytes synthesize albumin, which is the most abundant plasma protein and helps maintain the blood’s oncotic pressure, keeping fluid in the vessels. The liver also produces many clotting factors—such as fibrinogen and prothrombin—and a range of other plasma proteins like transferrin and ceruloplasmin that carry substances or have regulatory roles. While some proteins (like immunoglobulins) come from immune cells elsewhere in the body, the liver remains the main source of most plasma proteins. Other organs listed mainly handle immune function, filtration, or hormone production and do not serve as the primary producers of plasma proteins. That’s why the liver is the best answer.

Immunological memory rests in long-lived memory cells that respond rapidly upon re-exposure. The T cell side has its own memory population, memory T cells, which persist after the initial encounter and can quickly proliferate and become active effector cells again when the same antigen is encountered. This mirrors memory B cells in that both maintain a faster, more robust response on re-exposure, but for the cellular arm of the immune system. Cytotoxic T cells are primarily the immediate effectors that kill infected cells during the first wave of a response, rather than the long-term memory responders. Helper T cells coordinate and assist other immune cells during the response but are not defined by the memory function in the same way. Suppressor (regulatory) T cells regulate or dampen immune responses to prevent overactivity, not maintain memory. Thus, memory T cells best fit the role of a T cell derivative with a function similar to memory B cells.

Natural killer cells provide rapid, non-specific killing of virus-infected or transformed cells. They act without prior exposure or sensitization, unlike cells of the adaptive immune system. NK cells survey for signs of abnormal cells, particularly reduced expression of MHC class I or the presence of stress-induced ligands. When activating cues outweigh inhibitory signals, they unleash cytotoxic granules containing perforin and granzymes to induce apoptosis in the target cell. They can also be boosted by cytokines released during infection, enabling a quick response. This combination of rapid action and lack of prior exposure is what makes NK cells the correct choice for destroying virus-infected or tumor cells without prior encounter. Dendritic cells are primarily antigen-presenting cells that prime adaptive immunity, macrophages are phagocytes, and B cells are adaptive immune cells that produce antibodies.