Immune System

When you are asked to picture a human, what do you imagine? Someone young, strong, and healthy. When you imagine a human, you typically don't picture someone weak, frail and sick. That's courtesy of the immune system, whose function is to protect your body from the four categories of pathogens: bacteria, parasites, fungi and viruses. The immune system cells, leukocytes, can be classified into two separate categories: phagocytes and lymphocytes. Phagocytes consume invading organisms, and there are five different classes of phagocytes: basophils, neutrophils, eosinophils, monocytes and macrophages. Lymphocytes are capable of recognizing various pathogens, remembering them, and then destroying them whenever they invade. Lymphocytes can be classified into four primary groups: memory cells, killer cells, B lymphocytes and T lymphocytes. T lymphocytes can be further categorized as helper T cells and cytotoxic T cells. It is courtesy of lymphocytes that vaccines work: by injecting a weakened or dead form of a microorganism, the lymphocytes will recognize the pathogen when it attacks the body in full force, and the lymphocytes will then be able to fight them.

Types of pathogens

Types of leukocytes

Leukocytes and their functions

Key Immune System Terms:

Macrophage - A phagocyte that ingests invading organisms through phagocytosis, and destroys it by engulfing it into a lysosome.
Antigen - A foreign substance which invades the body and induces an immune response.
Helper T cell - A type of lymphocyte stored in the thymus that helps B cells destroy tagged antigens and signals phagocytes to perform their respective functions.
B cell - A type of lymphocyte stored in the bone marrow that tags invaders with antibodies to be destoryed by helper T cells.
Antibody - A type of protein manufactured by B cells and attached to antigens so that helper T cells can recognize and destroy the invading substance.
Killer cell - As their name suggests, killer cells are lymphocytes that consume invading organisms and either devour them or release chemicals which will destroy them.
Cytotoxic T cell - A type of lymphocyte that releases chemicals into a cell's plasma membrane, resulting in cytolysis. Cytotoxic T cells perform this function typically in cells infected by a virus before the viral DNA is replicated.
Memory cell - A type of lymphocyte that aids the function of antigen recognition.

Immune system cells

Immune system cells

Immune system cells

There are three primary types of immunity. The first, innate immunity, the immunity that we are born with. Some viruses or germs we are just naturally immune to, whereas other species would feel their effects. Our innate immunity also includes special immune cells, our mucus membranes and skin, which are our "first line of defense" when attacked by invading organisms. The second type of immunity is adaptive immunity, which develops as we grow, and includes lymphocytes which recognize invading organisms that we have been previously exposed to. The third type of immunity is passive immunity, is short-term immunity provided by another source. An example of passive immunity would be the antibodies passed from a mother's breast milk to her child.

Types of immunity

Two well-known immune system disorders are lupus and allergies. Lupus, made famous by the television show, House, is an autoimmune disorder. As in all autoimmune disorders, the immune system mistakes bodily tissues as foreign invaders and attacks them. Lupus is characterized by muscle soreness, joint pain and inflammation, and in some cases, attacks on the kidneys and other organs. Allergies are another abnormal immune system response, although this one is more common. Allergies occur when the immune system attacks antigens from our environment, and attack them as if they were pathogens. Allergic reactions can be characterized by a variety of responses, such as watery eyes, sneezing, itchiness, hives, swelling and anaphylaxis.