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Bone marrow is the main lymphoid organ where all blood cells including lymphocytes are produced. Both bone marrow and thymus provide micro-environments for the development and maturation of T-lymphocytes. After maturation the lymphocytes migrate to secondary lymphoid organs like spleen, lymph nodes, tonsils, Peyer’s patches of small intestine and appendix.

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Rheumatoid arthritis (RA) is a result of auto-immune reaction. In RA, immune system mistakenly attacks healthy self cells in our body, causing inflammation, painful swelling in the affected parts of the body.

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Acquired immunity is specific to pathogen. Innate immunity is non-specific type of defence present at the time of birth.

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Active immunity is slow and takes time to give its full effective response.

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Transplantation of tissue/organ often fails due to non-acceptance by the patient’s body. Therefore, tissue matching and blood group matching are essential before undertaking any graft/transplant. When the immune system recognises the protein in the transplanted tissue or organ as foreign, it initiates cellular immunity. As a result of this, there is a rejection of transplanted organs. To suppress the immune response during transplantation, histocompatibility antigen and immunosuppressants play an important role.

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Colostrum (Mother's first milk) rich in immunoglobulin A (IgA) antibodies are passed to newborn through the breast feed and provides natural passive immunity.

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Autoimmune diseases are certain unrelated disorders caused by inflammation and destruction of tissues by the body's own immune response. These disorders include acquired hemolytic anaemia, pernicious anaemia, rheumatoid arthritis, myasthenia gravis, psoriasis, vitiligo, etc. Alzheimer’s disease is a chronic neurodegenerative irreversible, progressive brain disorder that slowly destroys memory and thinking skills.

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MALT are significant aggregations of lymphoid tissues which are seen in relation to the mucosa of the major tracts like respiratory, alimentary canal and urinogenital tracts. It constitutes about 50 percent of the lymphoid tissue in human body.

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The Sabin vaccine or trivalent ‘oral polio vaccine’ consists of attenuated viral strains.

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Autoimmunity is a disorder of the body’s defence mechanisms in which an immune response is elicited against its own tissues, which are thereby damaged or destroyed. Autoimmunity may be caused due to genetic or environmental factors.

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Serum globulins are proteins that include gamma globulins (antibodies) and a variety of enzymes and carrier/transport proteins.

The specific profile of the globulins is determined by protein electrophoresis (SPEP), which separates the proteins according to size and charge. There are four major groups that can be identified: alpha-1 globulins, alpha-2 globulins, beta globulins, and gamma globulins. Once the abnormal group has been identified, further studies can determine the specific protein excess or deficit. Since the gamma fraction usually makes up the largest portion of the globulins, therefore antibody deficiency is mainly related with the low level of serum globulins.

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IgA immunoglobulins are the second most abundant class of immunoglobulins, which are mainly found in sweat, tears, saliva, mucus, colostrum and gastrointestinal secretions.

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Cell-mediated immune response (CMIS) consists of T-lymphocytes. It reacts against transplants. Transplantation may result in the rejection of the transplanted organs. The immune system recognises the protein in the transplanted tissue or organ as foreign and initiates cellular immunity against it.

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Each antibody molecule has four peptide chains, two small called light chains and two longer called heavy chains. Hence, an antibody is represented as ${\mathrm{H}}_2{\mathrm{L}}_2$.

In an antibody molecule, antigen binding site is located at N-terminal region.

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The primary lymphoid organs are bone marrow and thymus where immature lymphocytes differentiate into antigen-sensitive lymphocytes.

After maturation, the lymphocytes migrate into secondary lymphoid organs like spleen, lymph nodes, Peyer’s patches of small intestine and appendix.

These secondary lymphoid organ provide the sites for interaction of lymphocytes with the antigen.

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Innate immunity is non-specific type of defence, that is present at the time of birth. This is accomplished by providing different types of barriers to the entry of the foreign agents into our body. Acquired immunity is pathogen specific, characterised by memory cells.

Immune response mediated by B-lymphocytes is humoral immunity and other immune response mediated by T-lymphocytes is called cell-mediated immunity.