Human Immune System

Immunity is the resistance of an organism  towards foreign bodies that include pathogens. Immunity is provided by our body’s immune system which comprises of the lymphoid system, some body cells and chemicals. Immunity is of two types- innate and acquired. Innate immunity is non-specific and is present from birth in our bodies. It is the general protection provided to our body via various mechanical, chemical and other barriers (refer to article on ‘Innate Immunity’). Acquired immunity or adaptive immunity is specific in nature. We acquire this immunity on being exposed to foreign pathogens.

Our body’s way of protecting us through this immunity is using antigen-antibody complexes. Antigen is any foreign agent which is capable of entering the body, inducing production of antibodies and forming complexes with the same. Every antigen is however not a foreign body. Blood travels all over our body and familiarizes each part with other parts of the body. There are no blood vessels in the eye lens. Hence, the immune system treats its cells like antigens and attacks them. A similar case is observed in male scrotal sac. Antibodies are chemicals produced by cells of the immune system which bind with one or more antigens and thus rendering their action effectless. They thereby prevent infection. Antigens and antibodies are also called immunogens and immunoglobins, respectively.

Our Immune System

The immune system is divided into two parts – primary lymphoid system and secondary lymphoid system. Lymph is a connective tissue like blood which flows in lymph vessels, has a high WBC (white blood cell) concentration but is devoid of erythrocytes (RBCs- red blood cells).

The primary lymphoid system consists of bone marrow and thymus. Bone marrow is a matrix like material found in the bone cavity and is the site for haemopoiesis (formation of blood cells). In babies it is mostly pink in colour due to high erythropoiesis rate (formation of RBCs) . By the age of 30, most of it turns yellow in colour due to lymphocyte production. Later, it also produces fat cells. In old age, the bone marrow is practically non-existent. The following chart shows the steps of haemopoiesis.

Steps of haemopoiesis

steps of haemopoiesis

The thymus is a bi-lobed gland situated behind the sternum (breast bone) and just above the heart. T-lymphocytes mature here (B-lymphocytes originate and mature in bone marrow only). The T and B cells are the action cells of the immune system and produce antibodies. Thymus gland decreases in size with advancing age and degrades finally to a thin sheet like structure.

How does the immune system work?

The entrance point of any pathogen is the skin or mucosal cavities such as those of the respiratory tract (nose), gastro-intestinal tract (mouth) and urino-genital tract (external genital organs). If pathogens cause infection in the mucosal regions, they are protected by innate immunity. For skin tissue entrance, both immunities are in effect.

Skin has two layers- an upper epidermis and lower dermis. The epidermis has cells called dendritic cells which fuse with the pathogen due to Phagocytosis. These cells are macrophages. The antigen part of the pathogen’s body remains on the surface of these macrophages. They are called Langerhan’s cells in the skin and mucous membrane. On attachment of antigen to the surface, these cells release a chemical called IL-1  (inter leukine-1) which is a cytokine. Cytokine attracts response from innate immunity and also alerts the T-helper cells. T helper cells thus move from thymus to the lymph nodes and attach to a MHC-II (MHC= major histo-compatibility complex) molecule newly generated on the dendritic cell’s surface (dendritic cell has moved from affected tissue to lymph nodes which are part of the secondary lymphoid system along with the surface antigen). This attachment promotes the release of IL-2 (inter leukine-2) and B-cell promoting factor. IL-2 invites cytotoxic killer T-cells toward a newly generated MHC-I molecule on the Langerhan’s cell’s surface. It binds at the site and triggers a lysis action thus killing the infected cell. The B-cell promoting factor brings B-cells to the lymph tissue. Here, B-cells differentiate giving rise to plasma cells and memory cells. Plasma cells clone multiple copies of themselves and attack the free pathogens, thereby destroying them. Memory cells are a type of plasma cell. They too have no receptor on their bodies unlike their precursor (B-cell). They learn the structure of the pathogen and of the antibody used to destroy them. If the same pathogen attacks our body some other time in the future, they are destroyed immediately via memory cell response.

Antibiotics and Immunity

Antibiotics are simply chemical substances which are ingested during infections. They help cure the infection. Some antibiotics are pain killers while others are anti-pyretics. Pain killers suppress the neural response of the concerned area and comfort the patient. Anti-pyretics lower the body temperature. These have a role connected with immunity to the maximum.

Cytokines such as IL-1 and IL-2 are pyrogens i.e. they raise the body temperature. They act on the hypothalamus part of the brain which controls temperature of the body and maintains homeostasis. Normal body temperature is 98.3°F which optimum for enzyme activity. If the body temperature rises above 100°F, anti-pyretics are consumed. These have a structure analogous to cytokines and bind to the hypothalamus cytokine-receptors thereby preventing a further rise in temperature. Temperatures higher than 100°F though help kill pathogenous microbes also render body enzymes useless hence, antibiotics are necessary. Some antibiotics even carry chemicals specific to microbes. For example, a bacterium may have a resistance to tetracycline but not to ampicillin. In such cases, after diagnosis of microbe, the infected person is given a dosage of ampicillin. However, antibiotic courses have to be short. If they are longer than a few days, the bacteria or other pathogen may develop a resistance to that particular drug and therefore the disease will persist.

Vaccinations

Vaccinations are another matter. They are small quantities of microbes in a serum like solvent base. These microbes are called heptans. Their molecular weight is less than 1000 Da and they have just enough antigens on them to cause an immune response which results in production of antibodies but no antigen-antibody complexes. These heptans are incomplete antigens or immunogens. To release an immune response though, the microbes must have a certain molecular weight. Heptan weight is increased by using an amalgam. These incomplete antigens do not cause diseases but trigger an immune response simply so that memory cells are built against that pathogen. If there is any future exposure to it, the body will have a faster response. Vaccinations for tetanus, measles and hepatitis are common.

Recommendations for a healthy life

One must avoid exposure to pathogen containing material and must maintain personal hygiene. It is essential to be properly vaccinated against diseases and to get regular medical check-ups every six months. Also, trust your doctor. One must be free to discuss all problems with the doctor so a proper treatment can be designed.