The main parts of the immune system are:
- white blood cells
- complement system
- lymphatic system
- bone marrow
White blood cells
White blood cells are the key players in your immune system. They are made in your bone marrow and are part of the lymphatic system.
White blood cells move through blood and tissue throughout your body, looking for foreign invaders (microbes) such as bacteria, viruses, parasites and fungi. When they find them, they launch an immune attack.
White blood cells include lymphocytes (such as B-cells, T-cells and natural killer cells), and many other types of immune cells.
Antibodies help the body to fight microbes or the toxins (poisons) they produce. They do this by recognizing substances called antigens on the surface of the microbe, or in the chemicals they produce, which mark the microbe or toxin as being foreign. The antibodies then mark these antigens for destruction. There are many cells, proteins and chemicals involved in this attack.
The complement system is made up of proteins whose actions complement the work done by antibodies. Typically, we measure C3, C4, CH50 levels to help confirm the if someone has immune deficiencies.
The lymphatic system is a network of delicate tubes throughout the body. The main roles of the lymphatic system are to:
- manage the fluid levels in the body
- react to bacteria
- deal with cancer cells
- deal with cell products that otherwise would result in disease or disorders
- absorb some of the fats in our diet from the intestine.
The lymphatic system is made up of:
- lymph nodes (also called lymph glands) — which trap microbes
- lymph vessels — tubes that carry lymph, the colorless fluid that bathes your body’s tissues and contains infection-fighting white blood cells
- white blood cells (lymphocytes).
The Spleen is a blood-filtering organ that removes microbes and destroys old or damaged red blood cells. It also makes disease-fighting components of the immune system (including antibodies and lymphocytes).
The Bone Marrow is the spongy tissue found inside your bones. It produces the red blood cells our bodies need to carry oxygen, the white blood cells we use to fight infection, and the platelets we need to help our blood clot.
The thymus filters and monitors your blood content. It produces the white blood cells called T-lymphocytes.
Humoral (B-Cell) and Cell-Mediated (T-Cell) Immune Responses
The immune system distinguishes two groups of foreign substances. One group consists of antigens that are freely circulating in the body. These include molecules, viruses, and foreign cells. A second group consists of self-cells that display aberrant MHC proteins. Aberrant MHC proteins can originate from antigens that have been engulfed and broken down (exogenous antigens) or from virus‐infected and tumor cells that are actively synthesizing foreign proteins (endogenous antigens). Depending on the kind of foreign invasion, two different immune responses occur:
The humoral response (or antibody‐mediated response) involves B cells that recognize antigens or pathogens that are circulating in the lymph or blood (“humor” is a medieval term for body fluid). The response follows this chain of events:
- Antigens bind to B cells.
- Interleukins or helper T cells co-stimulate B cells. In most cases, both an antigen and a co-stimulator are required to activate a B cell and initiate B cell proliferation.
- B cells proliferate and produce plasma cells. The plasma cells bear antibodies with the identical antigen specificity as the antigen receptors of the activated B cells. The antibodies are released and circulate through the body, binding to antigens.
- B cells produce memory cells. Memory cells provide future immunity.
The cell‐mediated response involves mostly T cells and responds to any cell that displays aberrant MHC markers, including cells invaded by pathogens, tumor cells, or transplanted cells. The following chain of events describes this immune response:
- Self-cells or APCs displaying foreign antigens bind to T cells.
- Interleukins (secreted by APCs or helper T cells) co-stimulate activation of T cells.
- If MHC‐I and endogenous antigens are displayed on the plasma membrane, T cells proliferate, producing cytotoxic T cells. Cytotoxic T cells destroy cells displaying the antigens.
- If MHC‐II and exogenous antigens are displayed on the plasma membrane, T cells proliferate, producing helper T cells. Helper T cells release interleukins (and other cytokines), which stimulate B cells to produce antibodies that bind to the antigens and stimulate nonspecific agents (NK and macrophages) to destroy the antigens.