The Human Immune System. By M. Mohsin Ali Dynamo For MCAT Preparation. What exactly is Immunity?.
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By M. Mohsin Ali Dynamo
For MCAT Preparation
Background: In animals, in addition to physical barriers (like skin and the mucus membrane) and phagocytes (mainly macrophages and neutrophils) which create innate immunity, there is a third mechanism to defend their bodies against the foreign invaders; the immune system (or the adaptive/acquired immune system).
Components of the Immune System
The components of the Immune System include:
1. B Lymphocytes
2. T Lymphocytes
What are Antibodies? They are proteins called immunoglobulinssynthesized by the vertebrates in response to antigens that destroy the antigens or immobilize them. They are specific i.e. they cause only the destruction of that antigen which caused their production. They are actually synthesized/manufactured in B Lymphocytes from where they are secreted in blood and lymph.
What are Antigens? These are more or less proteins called immunogensthat stimulate the formation of antibodies.
Structure of an Antibody
Structure of an antibody:
An antibody consists of 4 PP chains—two identical light chains and two identical heavy chains linked by disulfide bridges. Variable amino acid sequences in the light and in the upper part of the heavy chains determine which antigen will bind to the antibody. In the same class, constant amino acids are same for all antibodies
T lymphocyte has been named thus owing to its relationship with the Thymus gland, which functions in making it immunologically competent. It is mainly involved in the Cell-mediated response.
B lymphocyte has been named due its relation with the Bursa of Fabricius, a lymphoid structure in the wall of clocoa of young birds from where B lymphocytes were first found to play a role in immunity. They are involved in humoral immune response and also play role in formation of immune memory cells.
The Cell Mediated Response
T-cells recognize the antigen, then they combat the micro-organisms. They also affect the rejection of foreign tissue during transplant. This response (explained briefly in figure) is called cell-mediated response.
Humoral Immune Response
The B cells recognize the antigens and form plasma cell clones. These clones synthesize and liberate antibodies in the blood plasma and tissue fluid. Here antibodies attach to bacteria and speed up phagocytosis or neutralize their toxins by producing antitoxins. This is the humoral immune response.
Bored, eh? Watch a movie! See next slide for it!
Types of Immunity
Vaccines are used to stimulate antibody production in organisms in response to a certain antigen (for which vaccine was made). The use of vaccine which makes a person immune to a disease is called artificially induced active immunity. In case the B memory cells trigger a natural immune response to a recurrent disease, the process is called naturally induced active immunity or the auto immune response.
If instead of injecting antigens to elicit an immune response, antibodies are injected in the body, the immunity that develops is called passive immunity. In the human body, the antigen-antibody complexes that form are destroyed by phagocytes. Antibodies are injected in the form of antisera, which are blood serum + antibodies. Passive immunity gives a short-lived response, as the antibodies that are injected are gradually used up and natural antibody production is not triggered.
This method of passive immunization is used to combat the active infections of tetanus (caused by soil bacteria Clostridium tetani), infectious hepatitis, rabies, snake bite venom etc. In case of snake bite venom, passive immunity is produced by the antitoxins—therefore the serum is called antivenom serum.
Vaccination: The administration of a vaccine (or antigenic material) for the purpose of inducing immunity (or developing adaptive immunity) is called vaccination.
For instance, if a person lives in a community where polio spreads in early childhood, he can be given an oral vaccine.
A vaccine is an attenuated culture of germs that can cause a mild form of the disease (not always) and thus trigger a humoral immune response.
Due to this response, B memory cells are produced from B lymphocytes. These memory cells can remain in the blood for an indefinite period of time, and when one is attacked by the disease again, these memory cells cause a secondary immune response.
The rate of a secondary immune response is undoubtedly higher than a primary immune response.
Pasteur’s experiment on chicken cholera bacteria was the key achievement that hallmarked vaccination, which had been discovered by Jenner in 1796.
The word vaccine comes from Latin vacca=cow, because the attenuated germs Jenner actually were causative agents of cow pox.
Whenever we get vaccination against a particular disease, we become immune to that infection or disease.
If we get vaccination against polio, measles, mumps smallpox etc. once in our lime time, we are protected from that disease.
Vaccination is not synonymous with immunization; immunization is the consequence of vaccination.
A little brain-teasing illusion to remind you that you aren’t done with Physics yet!