Defending against microbes

Adaptive immunity

When innate immunity is not enough, a second line comes into play — slower, but remarkably precise.

Two complementary systems

                    INNATE                   ADAPTIVE
------------------  ----------------------   ----------------------
delay               minutes                  several DAYS
specificity         none                     HIGHLY specific
memory              no                       YES
actors              phagocytes               B and T lymphocytes

The delay of adaptive immunity explains the typical duration of an infection: about a week, the time the specific response takes to build.

B lymphocytes: antibodies

B lymphocytes produce antibodies, Y-shaped proteins able to recognise one precise target, the antigen.

        antigen
           ▲ ▲
            Y            one antibody recognises ONE antigen,
           / \           as a key opens only one lock

Antibodies do not destroy directly: they neutralise and tag.

NEUTRALISATION  they block the virus, preventing it entering cells
AGGLUTINATION   they clump intruders together
OPSONISATION    they tag them, making phagocytosis easier

This is called humoral immunity — it acts in the body fluids, blood and lymph.

T lymphocytes: destroying infected cells

A virus sheltering inside a cell is beyond the reach of antibodies. The cell itself must then be eliminated.

   the infected cell displays viral fragments on its MHC
              |
              v
   the CYTOTOXIC T lymphocyte recognises it
              |
              v
   it perforates the membrane -> the infected cell dies

This is cellular immunity. A third type, the helper T lymphocyte, coordinates everything: it activates both B cells and cytotoxic T cells. It is precisely this cell that HIV destroys — which explains the global collapse of defences in AIDS.

Clonal selection

How can the body recognise microbes it has never met, including synthetic molecules?

It does not build the antibody to order. It produces in advance millions of different lymphocytes, each specific to a random antigen. When an intruder appears, it selects the matching one, and that lymphocyte multiplies.

   pre-existing repertoire :   ▲ ● ■ ★ ◆ ✚ ▼ ...   (millions)
                                  |
   antigen ●  ->  SELECTS        ●
                                  |
                          multiplication (cloning)
                                  |
                         ● ● ● ● ● ● ● ●

This mechanism, clonal selection, is a form of Darwinian selection inside the body: diversity is produced at random, the environment sorts. It is what allows a response to an antigen that never existed in the species' history.

Summary