The immune system is composed of white blood cells, the proteins they produce, and their actions in defending the body.
The innate immune system is in place to combat new threats, while the adaptive immune system is developed against specific, previously encountered threats.
Innate immunity provides the first line of defense against infection. Most of its components are present before infection and represent a set of disease-resistance mechanisms that are not specific to particular threats.
The skin provides a mechanical barrier of entry of microbes, while its low pH (3-5) slows microbial growth. Mucous membranes are covered with normal flora that competes with pathogens, while mucus traps organisms and cilia propel them from the body. Temperature and various soluble and cell-surface molecules contribute to innate immunity by impeding the growth of certain pathogens.
Phagocytic cells are responsible for ingesting pathogens and debris.
Other important cells of the innate immune system include:
Many of the molecules involved in innate immunity use pattern recognition to detect classes of molecules found on pathogens, such as lipopolysaccharides.
Adaptive immunity is responsible for the selective recognition and elimination of pathogens and molecules the body has previously seen. It is characterized by antigen specificity, diversity, immunologic memory, and self/non-self recognition.
An effective response involves two cell types - lymphocytes and antigen presenting cells.
Antigen-presenting cells include:
Lymphocytes are responsible for developed immunity.
T cells arise in the bone marrow and mature in the thymus gland. During maturation they express a unique antigen-binding molecule, the T Cell Receptor (TCR).