HIV infects and destroys CD4+ T cells, a major type of white blood cell involved in responding to viral diseases. As a result, the immune system is inhibited and prone to various illnesses. Pneumonia is another example.
When an antigen enters the body and B-cells detect it (either from prior exposure to the disease or via vaccination), antibodies are produced. When antibodies bind to an antigen (similar to a lock-key arrangement), it tells other components of the immune system to attack and destroy the invaders.
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