The spleen has two main functions. The first main function is immunological, while its second function is to filter unhealthy (senescent) red blood cells.

Most digestive enzymes are secreted by the pancreas into the small intestine. Systemic hormones come from a variety of glands throughout the body; the spleen does not have an endocrine function. White blood cells are synthesized in bone marrow and mature in the marrow and thymus. Serum proteins are synthesized in the liver.

The thymus is a lymphoid organ located in the mediastinal space. The thymus is the site of T-lymphocyte differentiation. The mature T-cells leave the thymus and migrate to the spleen, lymph nodes, and other lymphoid tissues where they control cell-mediated immune responses. The thymus grows from birth to puberty, at which point it begins to shrink. The reason for this involution may be that the organ has produced enough T-cells and is no longer necessary.

The spleen is another lymphocyte-producing organ. The spleen filters blood, exposing it to lymphocytes that destroy foreign particles. The size of the spleen remains constant, except in cases of infections such as mononucleosis. The tonsils are a patch of lymphoid tissue that contain lymphocytes located in the pharynx. The tonsils and adenoids form a ring of immunologically active tissue. These tissues remain at a constant size except when infected by bacteria. Lymph nodes receive lymph from a single organ or region of the body. An increase in size, known as lymphadenopathy, could result from combating infection or cancer. 

The spleen forms erythrocytes (red blood cells), and leukocytes (white blood cells, including lymphocytes) during the embryonic stage. After birth, only lymphocytes are produced.

The tonsils and adenoids are patches of lymphoid tissue located in the pharynx that filter pathogens that enter the body through the mouth and nose. Lymph nodes produce lymphocytes in response to infections by pathogens. The thymus is an organ that produces lymphocytes in infants and young children. 

The surface of each tonsil is covered with stratified squamous epithelium, which forms deep crypts that detect and respond to pathogens entering the body. The tonsils are located on either side of the throat at the back of the tongue.

Adenoids are lymphoid tissue located in the nasopharynx, in the midline at the back of the throat. The spleen is in the upper left quadrant of the abdomen. The spleen has a smooth surface, as it is covered by an outer capsule of connective tissue. The thymus is in the mediastinum between the lungs. The thymus is composed of two lobes containing multiple lobules divided into an outer cortex and an inner medulla. The thymus is the site of T-cell differentiation. Lymph nodes filter lymph and remove foreign particles. Lymph nodes are located throughout the body, and are concentrated in the neck, axilla, and groin. 

Lymph nodes function to drain lymph. As lymph carries interstitial fluid, it also carries pathogens and cancer cells. Malignant cells may spread through the lymphatic circulation.

Tonsils and adenoids are collections of lymphoid tissue in the pharynx that filter microbes that enter through the mouth and nose. They enlarge during infectious processes. The spleen filters blood, exposing it to macrophages and lymphocytes that destroy foreign particles and aged blood cells. The thymus is the primary site for T-cell differentiation. The mature T-cells leave the thymus and travel to the spleen, lymph nodes, and other lymphoid tissue where they control cell-mediated immune responses. 

The spleen is a lymphoid structure that contains resident lymphocytes that produce antiobodies, as well as T-cells that are released into the bloodstream. It also contains resident macrophages, which are responsible for removing and degrading microbes and worn-out red blood cells. 

The secondary response of the immune system is significantly shorter in duration due to the storage of memory cells after the initial infection has been combated. During the primary infection, a B-cell will bind with an antigen. Once this occurs, the B-cells will begin to divide rapidly into plasma cells and memory cells. Plasma cells release high quantities of antibodies, which are integral in combating the infection. Memory cells are stored in lymph nodes so that if the same antigen is ever encountered again, it can be quickly dealt with by a fast-responding production of the correct form of plasma cell. If lymph nodes were destroyed, memory cells would not be able to mount this quick secondary response.

The lymphatic system has many purposes, including draining the 10% of interstitial fluid that is not reabsorbed by the capillaries, housing the lymph nodes that produce B-cells, and serving as a migration site for macrophages that present antigens to B-cells to initiate an immune system reaction.

The bone marrow is responsible for producing red blood cells, and for allowing B-cells to mature.

The lymphatic system carries lymph, which is a colorless fluid containing white blood cells, to help the body remove excess interstitial fluid. Fluids from the blood leak out of the gaps between capillary epithelium and must be returned to circulation to prevent accumulation. The lymphatic system collects this fluid and passes it through lymph nodes, which house lymphocytes that screen the fluids for foreign antigens. The lymphatic vessels then carry the fluids to the heart, releasing the lymph back into circulation via the right atrium.

The liver is primarily responsible for the detoxification of the blood.

All other listed options are the primary roles of the lymphatic system. Digested fats are emulsified in the small intestine, then transported via lymph (rather than blood). They enter the blood stream through the subclavian vein. The lymph also contains a large number of lymphocytes, or white blood cells, which can screen for microbes. Excess interstitial fluid is transferred to the lymph via leaky capillaries. The thymus, a primary lymphoid organ, is responsible for T-cell maturation.