The first defense includes barriers to infection such as skin and mucus coating of the gut and airways, physically preventing the interaction between the host and pathogen. Pathogens which penetrate these barriers encounter constitutively expressed anti-microbial molecules that restrict the infection.
The second-line defense includes phagocytic cells, which includes macrophages and neutrophil granulocytes (polymorphonuclear leukocytes, PMN) that can engulf (phagocytose) foreign substances. Macrophages are thought to mature continuously from circulating monocytes.
Phagocytosis involves chemotaxis, where phagocytic cells are attracted to microorganisms by means of chemotactic chemicals like microbial products, complements, damaged cells and white blood cell fragments; chemotaxis is followed by adhesion, where phagocytes are sticked to microorganisms. Adhesion is enhanced by opsonization, where proteins like opsonins are coated on the surface of the bacterium. This is followed by ingestion, where phagocytes extends their projections, forming pseudopods that engulf the organism. Finally the bacterium would be digested by the enzymes in the lysosome.
In addition, anti-microbial proteins may be activated if a pathogen pass through the barrier offered by skin. There are several class of antimicrobial proteins, e.g. acute phase proteins (C-reactive proteins, for example, binds to the C-protein of S. pneumoniae - enhances phagocytosis and activates complement) and complement.
The complement system is a very complex group of serum proteins which is activated in a cascade fashion, and there are three pathways of activation - classical, alternative and lectin. Only alternative pathway is of significance here though - because classical pathway is activated by the adaptive immune system, the antigen-antibody complex - alternative pathway activates C1,C4,C2,C3,C5 and finally C6 to C9, which forms the membrane attack complex. Lectin pathway activates C2, C3, C4, and some C1 homologue calcium-dependent lectin family proteins.
Complement binding will result in cytolysis, chemotaxis, opsonization and inflammation.
Last but not least, interferon α and β are important for resistance to viral infection.
Adaptive immune system
The adaptive immune system, also called the acquired immune system, explains the interesting fact that when most mammals survive an initial infection by a pathogen, they are generally immune to further illness caused by that same pathogen. This fact is exploited by modern medicine through the use of vaccines. The adaptive immune system is based on immune cells called leukocytes (or white blood cells) that are produced by stem cells in the bone marrow. The immune system can be divided into two parts. Many species, including mammals, have the following type:
- The humoral immune system, which acts against bacteria and viruses in the body liquids (such as blood). Its primary means of action are immunoglobulins, also called antibodies, which are produced by B cells (B means they develop in the bone marrow).
- The cellular immune system, which takes care of other cells that are infected by viruses. This is done by T cells, also called T lymphocytes (T means they develop in the thymus). There are two major types of T cells:
- Cytotoxic T cells (TC cells) recognize infected cells by using T-cell receptors to probe the surface of other cells. If they recognize an infected cell, they signal the cell to become apoptotic ("commit suicide"), thus killing that cell and any viruses it is in the process of creating.
- Helper T cells (TH cells) interact with macrophages (which ingest dangerous material), and also produce cytokines (interleukins) that induce the proliferation of B and T cells.
- In addition, there are Regulatory T cells (Treg cells) which are important in regulating cell-mediated immunity.