2.3 Brian Pricked his Thumb and his Immune System Jumped into Action

Perhaps, we can answer the above question by describing what happens when five-year old Brian pricks his thumb during an apple-picking trip to an orchard. The pricking of Brian’s thumb constitutes a breach of the physical barrier that is part of the innate or natural immune system. In addition, the thorn that pricked Brain’s thumb would have transferred several hundred or so bacterial cells into the wound on Brian’s thumb. The initial response of Brian’s immune system will most certainly involve a type of white blood cell called a macrophage. These cells usually roam the body like sentinels looking for foreign invaders.

The macrophage is able to recognize the invaders as foreign and harmful because the invaders come bristling with PAMPs on their surfaces. Each PAMP can be recognized by the appropriate PRR on the macrophage. In simple terms, it is a bit like recognizing enemy troops by the uniform they wear. In addition, the invading bacterial cells actually produce chemical messengers that can be detected by macrophages: just like invading foreign troops communicating in a coded language on a particular radio frequency that can be picked up and decoded by United States soldiers as foreign. The net result is an aggressive onslaught on the colony of bacterial cells by an army of macrophages that actually eat up the invading cells. A few minutes after sustaining that wound, a casual observer would have noticed that the wound on Brian’s thumb has become red and swollen: a sure indication that an immune response is in progress! In addition to killing off some of the invading bacteria, the macrophages alert other cells of the immune system that there is an invasion in progress. After the macrophage has swallowed the bacteria, it chews it into tiny bits of protein and then deposits the pieces into the groove of a protein called major histocompatibility complex (MHC). Imagine the MHC molecule as the bun of a hotdog; the antigen to be displayed is placed in the groove where the sausage sits. There are two types of MHC proteins subdivided into class I and class II. Macrophages call on MHC class II molecules to shuttle the fragments of antigens to the surface of the macrophage so that they can be presented to the immune system. In this role, immunologists refer to the macrophage as a professional antigen-presenting cell (APC).

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