写在开头：这是本部分免疫学的第一个章节，主要总体概述一下人体免疫系统的主要架构。
Write at the beginning: This is the first chapter of this section, immunology. It provides an overview of the main structure and components of the body’s immune system.
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Vertebrate animals have three major lines of defense against pathogens:
1. Non-specific external barriers
This includes our skin, mucosal membranes, and some other structures that keep pathogens away from our body physically.
2. Non-specific internal defenses (innate immunity)
If pathogens successfully across the first external barriers, our body can induce the innate immune response to against pathogens. This mostly involves immune cells, such as macrophages, and proteins, such as interferons, to nonspecifically recognize and eliminate pathogens that enters the body.
3. Specific internal defenses (adaptive immunity)
If the innate immune response still cannot eliminate pathogens, our body will induce adaptive immune response to further target to pathogens specifically. This mainly includes humoral immunity and cell-mediated immunity, in which B cell and T cell play a main role.
Primary lymphoid organs are where immune cells develop and mature.
Bone marrow: hematopoetic stem cells give rise to all cell types in the blood.
Thymus: glandular organ near the heart where T cells mature.
Secondary lymphoid organs are where immune cells resides and respond.
Tonsil: traps ingested or inhaled microbes in the throat.
Spleen: filters blood to remove dead red blood cells and other microbes.
Lymph nodes: small organs throughout the lymphatic system that filter out dead cells and other molecules or microbes in the lymph.
Others: appendix, mucosa-associated lymphoid tissue (MALT), Peyer’s patches, etc.
* This is the sturcture of a lymph node. It is where immune cells gather and survey the surroundings to determine whether there is an infection to fight.
This diagram mainly shows the process of hematopoiesis, where all blood cells are generated from this process. Cells are divided to two main lineage in this process, for one is myeloid lineage, for the other is lymphoid lineage.
The myeloid lineage comprises most of the cells of the innate immunity.
1. Macrophages are professional phagocytes. They are also a type of antigen presenting cells (APCs). Tissue resident macrophages can engulf invading microbes. Besides, macrophages are differentiated from monocytes circulating in blood.
2. Mast cells are resident cells of connective tissue that contains many granules rich in histamine and heparin, which can mediate the allergic reaction.
3. Basophils are similar to mast cells, for both of them are granulocytes that rich in histamine.
4. Eosinophils are also a type of phagocyte. The main function of them is to against parasitic infection, and mediate allergic reaction.
5. Neutrophils are the most abundant white blood cells, and one of the first responders recruited to sites of infection. They can phagocytose the pathogens infected, degranulate to release antimicrobial cytotoxic, and even eject their chromatin and proteases as neutrophil extracellular traps (NETs) to trap bacteria.
6. Dendritic cells (DCs) are phagocytes and professional APCs. DCs can phagocytose pathogens, present the antigens on MHC molecules to T cells, activating the adaptive immune response. Thus, DCs serve as a crucial link between the innate and the adaptive immunity.
B cells and T cells in lymphoid lineage are respondsible for the adaptive immunity, while natural killer cells play a role in the innate immunity.
1. B cells are lymphocytes that originate and mature in the bone marrow. They can produce antibodies in response to foreign antigens. When encountering foreign antigens, naive B cells can be activated and differentiated into plasma cell and memory B cells.
2. T cells are lymphocytes that originate in the bone marrow and mature in the thymus. They are respondsible for cell-mediated immunity to against intracellular pathogens and tumor cells. There are three major types of T cells, including cytotoxic T cells, helper T cells, and regulatory T cells.
3. Natural Killer cells (NK cells) work primarily at own cells that have been infected by viruses or become cancerous. They kill cells by releasing cytotoxic granules containing perforin and granzymes, and also activating death receptors on target cells.
