The adaptive immune response is specific, and can be primary , if it occurs in the first contact with the antigen, or secondary, if it is due to subsequent contacts with the antigen. In addition, the immune response can be cellular (mediated by T lymphocytes) or humoral (mediated by antibodies).

Primary immune response

When antigen first enters the body, it generates a primary immune response. First there is a latency period in which no antibodies are produced, but after a few days IgM antibodies will appear in the blood until reaching a maximum at 10-15 days and later on almost disappearing.

Secondary immune response

If the antigen re-penetrates a second time, the secondary, more rapid, intense and prolonged immune response will occur. There will be almost no latency period, more antibodies (of the IgG type ) will be produced and will last much longer in the blood, even several years .

This immunological memory is based on lymphocytes, which after the first contact with the antigen, some transform into memory cells (B or T lymphocytes). These memory cells will be in the blood and lymphoid organs, and when they detect another new entry of the antigen, they will produce IgG antibodies that prevent the development of the infection. The duration of immunity depends on the lifetime of lymphocytes of memory and may be a few months or years to life, as in the case of smallpox, rubella or measles.