A few basics

Stem cells are immature, undifferentiated cells that have the ability to divide without limit and either replicate themselves or develop into specialised cells (e.g. blood, brain, muscle). The three types of stem cells are totipotent, pluripotent, and multipotent. Multipotent stem cells give rise to a small number of specialised cell types, including those of the immune system. They are produced in red bone marrow, the soft tissue found in the hollow shafts at the ends of long bones.

Leukocytes circulate in bodily fluids and blood and are either granular or agranular. Granular leukocytes have abundant tiny protein particles in their cytoplasm. These particles contain enzymes and other substances that respond to pathogens. Neutrophils, basophils, and eosinophils are all granular leukocytes.

Agranular leukocytes have some protein particles, but the particles are considerably less obvious and much smaller in number. Lymphocytes and monocytes are both agranular leukocytes.

Apart from cells, the immune system uses molecules, usually proteins, to help destroy micro-organisms and co-ordinate cell action by serving as messengers between them. Co-ordination can occur for cells in close proximity or at a distance.

Cells destined to become immune cells originate from multipotent stem cells in the bone marrow that divide and become either myeloid or lymphoid progenitor cells. Myeloid progenitors give rise to erythroblasts (that mature into red blood cells), megakaryoblast (eventually platelets), or myeloblasts. Myeloblasts develop into neutrophils, eosinophils, basophils, or monocytes. These immune cells respond quickly and nonspecifically to infection. The first three types of white blood cells are granular leukocytes; monocytes are agranular.

The lymphoid progenitor cells either emerge from the bone marrow as functioning B-lymphocytes or mature in the thymus gland and become T-lymphocytes. Lymphocytes either circulate in the lymphatic or blood vessels or settle in the lymph nodes and organs of the immune system.

Lymph is excess tissue fluid that flows through the lymphatic system. The lymphatic system is similar in structure to the circulatory system. In addition to lymphocytes, lymph carries some red blood cells and proteins and also picks up unwanted materials such as fat particles and bacteria. As lymph does this, it also bathes cells in oxygen and nutrients. Both blood and lymph vessels pass through organs such as the spleen and liver, areas where foreign materials the immune system has reacted against are destroyed.

Lymph nodes are small bean-shaped masses of lymphatic tissue surrounded by connective tissue and located throughout the lymphatic system. Areas with many lymph nodes include the neck (around the collarbone), armpit, abdomen, and groin. Swollen lymph nodes (sometimes called swollen glands) are an indication that the immune system is fighting off infection or disease. The lymph nodes produce and house lymphocytes as well as filtering and cleaning lymph before it is released into the bloodstream via the veins of the circulatory system.

Lymphocytes in blood, lymph, and lymphatic vessels carry out most of the work of the immune system, defending the body against infectious diseases, tumours, and foreign materials.

Complement responds in a nonspecific manner to defend a host against infections and is part of the innate immune system. Its name comes from its function, ‘complementing’ the work of antibodies in protecting the host. Inactive complement proteins circulate in the blood serum and plasma until the first complement component encounters an antibody-coated antigen. In a domino or cascade effect, one protein of the complement system sets off activity in the next protein in the system. The distinct major protein molecules are labelled C1 though C9 and each performs a different action.

The complement system includes nearly 30 different serum proteins, their cellular receptors, and related regulatory proteins. Complement enhances immune response by binding to antibody-coated microbes to attract phagocytes to the scene. It can also eliminate pathogens itself, as when a complement chain reaction produces a cylinder capable of puncturing the target cell membrane and causing cell death (immune cytolysis). Complement can also release chemicals to trigger or enhance an inflammatory response.