Dr Ira Shah
What is our immune system made up of ?
Our immune system is important for maintaining immunity in the body and preventing infections. It is made up of variety of body cells and proteins. Infact the first barrier to infection is the skin and the mucus membranes that line the nose, lungs and intestines. Our environment contains lot of bacteria and viruses. The skin and mucus membranes act as physical barriers and prevent entry of these germs into our body and internal organs. In case, a germ still gains entry into the body (when skin is broken or mucus membranes are inflamed) the various cells (White blood cells) and proteins (antibodies) help to fight the infection.

Which are the white blood cells that help to fight infection?
Our blood contains 3 types of cells - Red blood cells (they carry oxygen to various parts of the body), white blood cells (they consist of neutrophils, lymphocytes, monocytes, basophils and eosinophils that help to fight infection) and Platelets (cells that prevent severe bleeding whenever there is injury). The various white blood cells have different roles in maintaining immunity.

The neutrophils and monocytes (also called as phagocytes) primarily ingest and kill germs. Neutrophils can migrate into site of infection from the blood stream within minutes. They are responsible for formation of "pus". Neutrophils markedly increase during infection and thus lead to elevated white cell count.

Monocytes are present in blood stream but also line the walls of blood vessels in organs such as liver, spleen and lung. Monocytes in these organs are known as macrophages. They capture germs as they pass by in the blood and are especially useful for killing fungus and TB germs.

The lymphocytes mainly consist of B-lymphocytes, T-lymphocytes and Natural Killer Cells (NK cells). B-lymphocytes produce antibodies called as immunoglobulins (they are highly specialized proteins that bind with a germ and make it inactive or making it easier for the phagocytes to kill the germ) B-lymphocytes can produce antibodies against virtually all organisms. T-lymphocytes do not produce antibodies but directly kill viruses, fungus and certain bacteria. Certain T-lymphocytes called as Helper T-cell assist B-lymphocytes in producing antibody and help killer T-lymphocytes in to kill germs. Natural killer cells kill cells infected with viruses especially the herpes virus, Epstein Barr virus and Chicken pox virus.

How are the various white blood cells formed in the body?
B-lymphocytes develop from stem cells in the bone marrow. T-lymphocytes also develop from stem cells in the bone marrow. In early fetal life, the immature T-lymphocytes migrate to the thymus and develop into mature T-lymphocytes. Mature T-lymphocytes then leave thymus and are found in spleen, lymph nodes, bone marrow and blood. If fetus does not have the thymus, then mature T-lymphocytes cannot develop. NK cells and phagocytes also develop from stem cells in bone marrow.

What are the various kinds of antibodies formed by the B-lymphocytes?
There are 4 major classes of antibodies
1. Immunoglobulin G (IgG)
2. Immunoglobulin A (IgA)
3. Immunoglobulin M (IgM)
4. Immunoglobulin E (IgE)

Another type of antibody formed is IgD. Each antibody has specific structure and specific action. IgA antibodies are present in mucus membranes and help to protect infection in the respiratory tract and intestines. IgM antibodies are first antibodies formed in response to infection followed by formation of IgG. IgE antibodies are responsible for allergic reactions.

Which are other components of the immune system?
Another component of the immune system is the complement system. The complement system is composed of 30 proteins produced by the liver in inactive form. In contact with germs, complement system is activated and coats the germs which make it easier for phagocytes to ingest them.

Which are important organs important for immune system?
Thus, the organs important for immunity are thymus, bone marrow, liver, spleen, blood and lymphnodes. Tonsils are also important as they store lymphocytes in them.

Contributor Information and Disclosures

Dr Ira Shah
Consultant Pediatrician, Nanavati Hospital and B.J.Wadia Hospital for Children, Mumbai India.

First Created : 1/2/2002

