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Clinical Overview of the Immune System

The immune system works hard to defend you from pathogens, including bacteria and viruses. It’s made up of leukocytes (white blood cells), proteins, and other tissues, including the lymphatic system. When it’s not fighting infection, the lymphatic system is busy draining excess fluid from the body’s tissues and removing debris from that fluid.

Beginning with red bone marrow and leukocytes

Red bone marrow is the soft part inside certain bones. Red bone marrow makes red blood cells and platelets that are important for the cardiovascular system, and it also makes leukocytes, which are part of the immune system.

Different types of leukocytes exist: lymphocytes, which identify and remember enemy microorganisms to help the body destroy them, phagocytes, which chew up those microorganisms, and basophils, which are involved with allergies and inflammation.

Fighting infection with lymphocytes

One group of leukocytes consists of lymphocytes, and you can break them down into two types: B cells and T cells. After they’re born in the red bone marrow, some lymphocytes stay in the bone and others leave to seek out the thymus.

  • B cells: Cells that stay in the bone marrow to fully mature

  • T cells: Cells that travel to the thymus

So what exactly do B cells and T cells do? They start the war against antigens, foreign invaders such as bacteria or viruses in the body. When antigens are detected by any of the lymphocytes, the B cells are stimulated to produce antibodies, which are proteins that attach themselves to those antigens. Together they form complexes called the antigen-antibody complexes. Although the antibodies find the antigens, they can’t kill them, so the T cells come in to help. The T cells call in phagocytes to help finish off the invaders, and then the phagocytes help clean up.

The antibodies stay in your body, so if the antigens they’re specifically targeted for return, they’re ready to destroy them as soon as they show up. You become immune to diseases such as chicken pox and measles through this sensitization.

Binging on bacteria with phagocytes

Phagocyte means a cell that eats. Phagocytes, a type of white blood cell, engulf foreign particles such as bacteria by a process called phagocytosis. Phagocytes come in three types:

  • Neutrophils: This type of phagocyte is the most common. They’re small and contain granules, so they’re sometimes referred to as neutrophil granulocytes. They eat up (or in other words, phagocytize) bacteria.

  • Eosinophils: These phagocytes are also granulocytes. They destroy the complexes formed by antigens and antibodies. Eosinophils are also important in fighting foreign invaders like parasites. They’re also active during allergic reactions.

  • Monocytes: These cells are immature macrophages (large cells that eat). A large number of monocytes are stored in the spleen. The macrophages phagocytize enemy cells and produce cytokines (cell-signaling proteins) that communicate with other cells. They also help T cells recognize antigens.

Controlling histamines with basophils

One other type of white blood cell isn’t a phagocyte or lymphocyte (but is a granulocyte). It’s called the basophil, and it’s involved in inflammatory reactions, allergies, and parasitic infections. Basophils release histamines that increase blood flow to tissues, but they’re also responsible for some of the symptoms of allergies.

Allergies are an exaggerated immune response to allergens. Allergens, like antigens, are foreign substances, but they’re generally not harmful. In a person with allergies, the allergen stimulates an immune response that is triggered by the release of the histamine from basophils and eosinophils (a type of phagocyte). Histamine causes the symptoms associated with allergies, including itchy, watery eyes and a runny nose. Antihistamine drugs attempt to block the effects of histamines.

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