Your immune system recognizes infectious microorganisms and mounts protective responses against the damage they would otherwise cause. Immune responses involve functionally distinct types of white blood cells (leukocytes). Leukocytes develop from pluripotent hematopoetic stem cells in bone marrow through two lineages: myeloid and lymphoid. The myeloid lineage comprises cells of innate immunity (macrophages, granulocytes, mast cells and dendritic cells) that are mainly important during the unspecific phase of immune responses (within first 96 hours after microbial invasion). The lymphoid lineage comprises natural killer cells and lymphocytes (T and B cells). Lymphocytes form the adaptive branch of immunity and are activated only by specific structures of a given microorganism. Sufficient numbers of specific lymphocytes are generated about 96 hours after first invasion of a microbial pathogen; on subsequent encounters they react more efficiently due to immune memory.
Macrophages
After leaving bone marrow, macrophages circulate in peripheral blood as monocytes and further differentiate in peripheral organs where they reside for a long time. They engulf and destroy invading microorganisms (a process called phagocytosis) and clear remnants of dead cells at inflamed sites. They also present fragments of the mircrorganisms to a subset of lymphocytes and activate adaptive specific immune responses.
Granulocytes
Granulocytes are called also polymorphonuclear cells, due to their oddly shaped nuclei. They have three subtypes distigushed by different staining characteristics of their cytoplosmatic granules (neutrophils, eosinophils and basophils). Neutrophils are most numerous, and their granules contain anti-microbial substances. They are phagocytes. In contrast to macrophages, their life span is only several days. What roles eosinophils and basophils play in protective mechanisms is less well understood. They seem important in defense against parasites that are too large to be engulfed by macrophages (such as worms). Their contribution to allergic immune response is rather more damaging than protective.
Mast Cells
Mast cells are another cell type of myeloid lineage involved in anti-parasitic and allergic response. Similarly to macrophages, they undergo their final differentiation in peripheral organs. Mast cells seem to protect internal surfaces of your body (airways, gastrointestinal tract and blood vessels) by way of the contents of their cytoplasmic granules, including histamine, heparin, lipid mediators, cytokines and enzymes.
Dendritic Cells
Dendritic cells are the third type of phagocytic cells. They resemble dendrites on neuronal cells and hence their name. Immature dendritic cells leave bone marrow, circulate in the blood stream and enter peripheral organs. Upon encounter with antigens, they mature. Their major function is not clearance of the invading microorganisms but presenting microbial fragments to a subset of lymphocytes.
B Lymphocytes
B cells develop in bone marrow and then appear in peripheral blood, lymph nodes, mucosal lymphoid tissue or the spleen. Their activation occurs after binding of a specific antigen on an invading microorganism to B cell receptors. This induces B cells to proliferate and differentiate into plasma cells, secreting specific antibodies (immunoglobulins). Antibodies have several functions; for example, they neutralize soluble microbial toxins or coat the microorganisms. Antibody-antigen complexes then activate a protein complement cascade, which leads to formation of a complement lytic complex that destroys the coated microorganism.
T Lymphocytes
This subset of lymphocytes also originates in bone marrow but needs to migrate to the thymus, where the final stages of development of T cell subsets occur. T lymphocytes need to interact with a major histocompatibility complex on cells, which present degraded microbial fragments to the T cell receptor. Type 1 complex is on all body cells, while Type 2 complex is only on antigen-presenting cells (such as dendritic cells or macrophages). There are three functionally distinct types of T cells: cytotoxic T cells (that are activated by Type 1 complex and kill cells infected with viruses), helper T cells (that are activated by Type 2 complex and provide signals to efficiently integrate the immune responses, for example, by providing molecules facilitating B cell responses) and regulatory T cells (that control the immune response by suppressing activities that could become harmful to the host's own tissue). Within these subtypes, additional subgroups continue to be identified.
Natural Killer Cells
The common lymphoid progenitor also gives rise to lymphoid-like natural killer cells that do not have antigen-specific receptors (and, as such, contribute to the innate part of the immune system). Natural killer cells are activated by soluble factors and interferon and are able to kill virus-infected cells or tumor cells.
References
- "Janeway's Immunobiology"; Kenneth Murphy, Paul Travers, Mark Walport; 2008
