The lymphatic system is responsible for removing excess fluid from the interstitium and returning it to the bloodstream. The fluid will travel through lymph nodes, which monitor for the presence of microbes in the blood. If microbes are present, the lymph nodes will mount an immune response. The lymphatic system also reroutes fat digestates from the small intestine's capillaries and drains them into the veins of the neck. Lymphatic vessels do not assist in the transfer of oxygen to body tissues. 

Red blood cells generally do not leave the circulatory system; thus there are very few opportunities for lymphatic vessels to pick up red blood cells in the extracellular space.

The lymphatic system plays a key role in transporting immune cells, such as lymphocytes, throughout the body. When fats are emulsified and digested in the small intestine, they are transported into the lymph rather than into the blood. When plasma and fluids leak out of capillaries, the lymphatic system is responsible for collecting and returning these fluids to circulation. A fault in this function of the lymphatic system results in edema.

Some viruses evade the immune system using a mechanism known as latency. The virus essentially becomes dormant, usually in cells such as neurons that have few major histocompatibility (MHC) class I molecules. The combination of limited to no active viral replication combined with residence in cells with few MHC lass I molecules allows the virus to evade routine immune monitoring.

When conditions arise, such as fever due to infection caused by another pathogen or emotional stress, the virus can becomes reactivated and can result in an outbreak.

T-cells generally serve to recognize antigens and escort them to B-cells to elicit a response (helper T-cells) or to recognize antibodies bound to infected cells (cytotoxic T-cells). B-cells, in contrast, will directly secrete antibodies against the antigen after binding of the antigen at the membrane.

Both types of cells have receptors that can bind to antigens, allowing for recognition and communication. Other commonalities include the presence of a transmembrane domain and the presence of a variable region, which will allow for specificity in antigen binding.

The similarities are mainly structural, and the receptors have many functional differences. T-cell receptors will only bind fragments of antigens, while B-cell receptors will bind full molecules. Only T-cell receptors will interact with major histocompatibility complexes (MHC). Though the structure of both receptors types is determined by peptide chains, B-cells use heavy and light chains, while T-cells use alpha and beta chains.

The spleen plays a key role in the immune system as one of the blood filtration centers of the body. It is also involved as a lymphatic organ, allowing filtrates from the blood to be distributed to the body via lymph.

Macrophages and neutrophils are the major cells of the immune system responsible for phagocytosis. Phagocytosis means ingestion of material. Therefore these cells work as "garbage collectors" within the body. Cells destined for destruction are tagged for phagocytosis by special molecules.