Immune memory: primary and secondary responses.

• Acquired immunity is pathogen-specific and has memory.
• The first encounter with a pathogen gives a primary response of low intensity. A later encounter with the same pathogen gives a much stronger secondary, or anamnestic, response, as if the body remembers the first meeting.
• Both responses are carried out by two kinds of lymphocytes in the blood: B-lymphocytes and T-lymphocytes.
• B-lymphocytes release proteins called antibodies into the blood to fight pathogens. T-cells do not secrete antibodies themselves but help B cells produce them.
• An antibody has four peptide chains, two small light chains and two longer heavy chains, so it is written H2L2. IgA, IgM, IgE and IgG are some of the antibodies our body makes.
• Because antibodies are found in the blood, the antibody-mediated response is called the humoral immune response.
• The second type of acquired response is cell-mediated immunity (CMI), carried out by T-lymphocytes.
• Organs such as heart, eye, liver or kidney can be transplanted only after tissue matching and blood group matching, and the patient still takes immuno-suppressants for life. The body tells 'self' from 'non-self', and CMI is responsible for graft rejection.

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