A single gram of soil can contain more than a billion microorganisms. microorganisms are everywhere, ready to invade our bodies. Nevertheless, most of us are healthy most of the time. so...Why are we not sick all the time?

The immune system is our defense system against disease, It is made up of a lot of different organs, cells, and proteins and is essential for our survival. The immune system' s main tasks are neutralizing harmful pathogens and the body's own particular sick cells (like cancerous cells), it also recognizes and fights harmful substances from the environment.

The immune system can be classified into two subsystems: the innate immune system (nonspecific) and the adaptive immune system (specific ).

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The Innate Immune System

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Unlike the adaptive immune system, the innate immune defense mechanisms are not specific to a particular pathogen. These defense mechanisms come into play shortly after a pathogen appears in the body. Anatomical barriers are the first line of defense.

Anatomical barriers 

These barriers can be physical, chemical and biological. among the physical barriers mucosas (mucous membranes) and the skin which prevents pathogens from entering. Chemical barriers include organic acids, enzymes, and mucus.

White blood cells

White blood cells (leukocytes) are the cells of the immune system that are involved in protecting the body.All white blood cells are produced in the bone marrow. Leukocytes can be classified into types distinguished by their physical and functional characteristics.

Among these types mast cells, phagocytes and natural killer cells.

Mast cells:

Mast cells are basically the 'regulators' of the immune system, each mast cell contains a storage called secretory granule containing powerful active molecules called mediators. these mast cells modulate many aspects of physiological and pathological conditions, when activated these regulators release their active molecules, among these mediators an organic nitrogenous called histamine which causes the signs of the inflammation and recruits phagocytes.

Phagocytes:

Phagocytes 'eating cells' are the immune cells that engulf pathogens by wrapping its membrane around it and then realizing lysosome that kills and digests the pathogen. these cells are like your very own police patrol unit, patrolling your body searching for pathogens.

Among phagocytes dendritic cells, macrophages, and neutrophils

Natural killer cells (NK cells):

Unlike phagocytes, natural killer cells don't attack pathogens but host cells, for example, virus-infected cells, these natural killer cells don't require activation in order to attack a cell, therefore, they were named 'natural killer'.

Here is a picture of major types of immune cells

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Adaptive immune system

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The adaptive immune system evolved in early vertebrates thousand of years ago to allow a stronger immune response as well as immunological memory.

Each substance that enters the body is recognized via specific molecules called Antigens which represent some sort of identity.Antigens are identified by immune cells and the response can be either:

-A tolerance specific to self-recognition.

-An inflammation and initiation of an immune response specific to exogenous antigens.

The adaptive immune system cells are essential, antigen presentation cells and lymphocytes T and B.

Antigen-Presenting Cells:

An antigen-Presenting Cell (APC) is a cell that displays antigen complexed with specific molecules called major histocompatibility complexes (MHC) on their surface and present them to T cells.

The process begins with an entrance of a foreign particle to the body it can be a germ, virus, fungi or even a toxin.APC recognize foreign antigen, phagocyte them and display them on their surface, a phenomenon called antigen presentation.

There are two types of APC, ordinary APC represented by almost all human cells.For example, in case of an infected respiratory cell, antigens are displayed in form of MHC I to induce a cytotoxic response to cause their own death in order to limit the spread of the disease.The other type is professional antigen-presenting cells including macrophages, B cells, and dendritic cells.These cells are specialized and can induce two different immune responses.

Lymphocyte T (T-cells):

Once formed in the bone marrow, T progenitor cells migrate to the thymus mature and become T cells.All T cells express TCRs (T cell receptors) and other receptors.

The most important are CD4 and CD8 which separate T-cells into two types, CD8 cytotoxic T-cells, and CD4 Helper T-cells.

Helper T cells help with the activation of B cells, and other immune cells.

Cytotoxic T is responsible for removing intracellular pathogens and damaged cells.

T cell receptors can only recognize antigens that are bound to Major Histocompatibility Complex class 1 (MHC I) or class 2 (MHC II).

Lymphocyte B (B-cells):

B-cells are formed and matured in the bone marrow.They express membrane-bound antibodies.Therefore when encounters an antigen that fits, B-cells quickly divides in order to become either a memory B cell or Plasma cells.In fact, this is why vaccination is effective because the second immune response to the same pathogen is usually stronger and more efficient(IgG antibodies>IgM antibodies).

Plasma cells originate from one B-cell, they secrete large amounts of antibodies specific to the antigen.Antibody and antigen reaction facilitates phagocytosis by other immune cells, complement activation and hide nude antigen sites, thus sterilizing their action.

Humoral vs Cell-Mediated Immunity:

Let's say a bacteria enters your body after an injury, Antigen-Presenting cells recognize them and display their antigen on their surface with an MHC.Then they present it to T cells which lead to a proliferation of Helper T-Cells.In fact, Helper T-Cells are bandmaster of this whole process.They secrete specific molecules called cytokines needed for communication between cells.There are different types of cytokines IL2, IL12, IL3 etc...

Based on the type of cytokines secreted, two types of reaction can happen.

Either a humoral response (humoral=fluid) which lead to a proliferation of B-cells and increased level of antibodies.With that being said, we now know that humoral immunity can be passed from mother to baby through milk (rich in antibodies).

Either a cell-mediated response which leads to cytotoxic T-cell proliferation and Natural killer+Macrophage activation.This immune response is mainly to get rid of more intracell pathogens and malfunctioning cells.Thus, Cell-mediated immunity can be acquired through T cells from someone who is immune to the target disease or infection.

References :  

https://www.khanacademy.org/test-prep/mcat/organ-systems/the-immune-system/a/adaptive-immunity

https://www.ncbi.nlm.nih.gov/pubmedhealth/PMHT0025680/

https://www.webmd.com/a-to-z-guides/components-of-the-immune-system

https://www.ncbi.nlm.nih.gov/books/NBK10752/

Picture Credit:

Thumbnail - pixabay.com

T cells picture - Flickr.com

All the rest -commons.wikimedia.org

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