Our immune system is one of nature's most fascinating inventions. It easily protects us against billions of bacteria, viruses and other pathogens. Most of us do not realize on a daily basis that our immune system is constantly on the alert to counterattack at the first signal/symptom of the invasion of harmful microorganisms.

The immune system is extremely complex. It consists of many organs, many types of cells and proteins that have different tasks to fight foreign "invaders".

Self and non-self

In humans, all nucleated cells express distinctive surface molecules called major histocompatibility complex class I (MHC class I) that identify them as being self. Anything that does not posses these “self tags” may be recognized by the immune system as foreign and targeted. Anything that triggers the immune system is called an antigen.

The Complement System

The first line of the immune system that encounters invaders, such as bacteria, is a group of proteins called the complement system. These proteins float freely in the blood and can quickly reach to the affected area, where they can react directly with antigens – molecules recognized as foreign.

When proteins of the complement system are activated, they can:

● Trigger/launch the inflammatory response
● Attract voracious cells such as macrophages to the site
● Adhere¹ to the "intruders" so that the phagocytes can more easily destroy them
● Destroy "intruders"

Phagocytes

Phagocyte is type of cell that has the ability to ingest, and sometimes digest, foreign particles, such as bacteria, carbon, dust, or dye. It engulfs foreign bodies by extending its cytoplasm into pseudopods (cytoplasmic extensions like feet), surrounding the foreign particle and forming a vacuole. Poisons contained in the ingested bacteria cannot harm the phagocyte so long as the bacteria remain in the vacuole; phagocyte enzymes are secreted into the vacuole in which digestion takes place.  There are three main types of phagocytes: neutrophiles, macrophages and dendritic cells.

	Neutrophiles   – a type of immune cell found in blood. They often constitute the first line of defense during infection. They attack all "intruders" and "bite" them until they destroy them. Pus from infected wounds usually contains dead granulocytes. In addition, a small part of the granulocyte population is specialized to attack larger parasites such as worms.
	Macrophages ("big eaters") respond to invasion slower than granulocytes, but they are larger, live longer and have much greater capabilities. Macrophages also play a key role in alerting other parts of the immune system2. They come from white blood cells called monocytes, which transform into macrophages when they pass from the blood into the tissues.
	Dendritic cells – A special type of immune cell that is found in tissues, such as the skin, and boosts immune responses by showing antigens on its surface to other cells of the immune system. A dendritic cell is a type of phagocyte and a type of antigen-presenting cell (APC). And like macrophages, dendritic cells help activate the entire immune system2. They can cleanse body fluids of foreign particles and microorganisms.

Lymfocytes – T cells and B cells

Lymphocytes are white blood cells (leukocytes) and come from the bone marrow, but they migrate to different parts of the lymphatic system, such as the lymph nodes, spleen or thymus. There are two main types of lymphocytes: T cells and B cells. The lymphatic system also includes a transport system - the lymphatic vessel system - used to transport and store lymphocytes. The lymphatic system supplies lymphocytes to our body and filters tissues from dead cells and microorganisms that have attacked us, such as bacteria.

On the surface of each lymphocyte there are receptors that enable them to recognize foreign substances. These receptors are very specialized and only match one specific antigen. To understand how such specific receptors work, think of a hand that can only grasp one type of object, for example only an apple. Such a hand would be a true master at picking apples, but it would be unable to grab anything else. In our body, such a single receptor would correspond to the hand that captures its "apples". Lymphocytes travel throughout our body until they encounter an antigen that has the right shape and size to match their specific receptor. It seems that perhaps the fact that each lymphocyte's receptors can only match one specific type of antigen would be a limitation, but the body copes with this by producing so many different types of lymphocytes that the immune system can recognize almost any invader.

T Lymphocytes

T lymphocytes (T cells) form two main and distinct groups: T helper lymphocytes and T killer lymphocytes. The name T lymphocytes comes from the Latin name of the thymus – thymus – a gland located behind the sternum. T lymphocytes are produced in the bone marrow and then migrate to the thymus where they mature.

Th helper lymphocytes are the main driving force and regulator of the immune system. Their primary task is to activate B lymphocytes and killer T lymphocytes. However, the Th helper cells themselves must be activated first. This happens when a macrophage or dendritic cell that has previously absorbed the intruder moves to a nearby lymph node and presents information about the caught pathogen. The phagocyte presents a fragment of the intruder's antigen on its surface in a process known as antigen presentation. A Th helper cell is activated when its receptor recognizes an antigen. Once activated, the Th helper cell begins to divide and produce proteins that activate B and T cells as well as other cells of the immune system.

Antigen Presentation

The presentation of antigens is the task of antigen presenting cells. These include phagocytes, primarily dendritic cells (derived from macrophages) and macrophages. Their main task is to present the collected antigens. These cells present foreign antigen to other immune cells, and secrete pro-inflammatory cytokines that attract cells of the adaptive immune response. 

Natural Killer Cells (NK cells) specialize in attacking body cells infected with viruses and sometimes bacteria. It also attacks cancer cells. The killer T cell has receptors that look for any matching cell. If a cell is infected, it quickly dies. Infected cells can be recognized by tiny traces of the intruder - the antigen that can be detected on their surface.

B Lymphocytes

The B cell (B Lymphocyte) searches for an antigen that matches its receptors. If it finds such an antigen, it attaches to it and a trigger signal is activated inside the B lymphocyte. But to be fully activated, the B lymphocyte also needs a protein produced by Th helper lymphocytes. When this happens, the B lymphocyte begins to divide, producing its own cell clones, and during this process, two new types of cells are created: plasma cells and memory B lymphocytes.

The plasma cell is specialized in producing specific proteins called antibodies that will act on an antigen that fits the B cell receptor. Antibodies released by plasma cells can seek out "intruders" and help destroy them. Plasma cells produce antibodies at an extraordinary rate and can release tens of thousands of antibodies per second. When Y-shaped antibodies encounter a matching antigen, they attach to it. The attached antibodies serve as a "tasty coating" for feeding cells such as macrophages. Antibodies also neutralize toxins and disable viruses, preventing them from infecting new cells. Each arm of the Y-shaped antibody can attach to a different antigen. So when one arm attaches to one antigen on one cell, the other arm can attach to another cell. In this way, pathogens are gathered into larger groups, which are easier for phagocytosing cells. In addition, bacteria and other pathogens covered with antibodies are easier targets for attack by complement system proteins.

Memory B cells Memory lymphocytes can recognize an antigen introduced into the body during a prior infection or vaccination. Memory lymphocytes mount a rapid and strong immune response when exposed to an antigen for a second time. Both T lymphocytes (T cells) and B lymphocytes (B cells) can become memory cells.

Figure 1: B-cell–T-cell interactions.The two-way interaction between B cells and T cells provides the basis for the concept that, in certain autoimmune diseases, an amplification cycle might allow persistent immunopathology to arise from a minor 'trigger' factor. Such a trigger might initiate the cycle through events in either the B-cell or the T-cell compartment, including the stochastic generation of both B-cell receptors (BCRs) and T-cell receptors (TCRs).

Innate Immunity and Adaptive Immunity

Innate immunity is the body's first line of defence against pathogens. It is general and non-specific, which means it does not differentiate between types of pathogens. Adaptive immunity is a type of immunity that is built up as we are exposed to diseases or get vaccinated.

Innate immunity, also known as genetic or natural immunity, is immunity that an organism is born with. This type of immunity is written in one’s genes, offering lifelong protection. It is considered the more evolutionarily primitive immune system and consequently, as well as being found in vertebrates, is also found in various shapes and forms in plants, fungi and insects. The innate immune response is fast acting and non-specific, meaning it does not respond differently based on the specific invader that it detects.

We are not born with adaptive immunity and it is not “hard wired” in their genes like innate immunity. It is acquired during their lifetime as a result of exposure to specific antigens, be that through natural means such as infection or by vaccination. Consequently, it is also known as acquired immunity. An adaptive immune response is much slower than an innate response, taking days or even weeks to develop on first encounter (the primary immune response), but is specific to the antigen(s) present and can retain a long term “memory” to enable a faster response if it is encountered again in the future. Adaptive immunity does it necessarily last throughout an organism’s entire lifespan, especially if it is not regularly re-exposed, although it can.