Advanced · Human Biology

The Systems Series: Immunity

Immunity is a state of the body that provides us with protection against diseases. A disease can be defined in this post here. Immunity can be separated into two main flavours; innate immunity and adaptive immunity. There are many differences between these two responses to infection, but both responses work together to eliminate the pathogen causing disease from the body.

Innate immunity provides the body with an immediate response to invading pathogens entering the body. The innate response is non-specific; it targets all pathogens rather than specific ones, which we see in adaptive immunity. The first line of defence provided by innate immunity is physical barriers against infection. This includes your skin, mucous in your nose and throat, and tears in your eyes. Physical barriers prevent pathogens from entering the body completely.
So, what happens when these barriers do not work? The cellular innate immune response comes into play. White blood cells, shortened to WBCs, are involved in this type of response. There are so many different types of WBCs that all play different roles in immunity, but for this post we will be focusing on phagocytes.

Phagocytes will protect against infection by ingesting harmful pathogens in a process known as phagocytosis. These WBCs become activated upon contact with inflammatory mediators such as bacterial products, which causes the phagocytes to produce more receptors on their cell surface. The receptors will allow the phagocyte to stick to capillary walls which promotes their movement towards the site of infection, increasing their likelihood of encountering a pathogen. Other receptors, known as pattern-recognition receptors (PRRs) will recognise a protein on the surface of a pathogen; this protein is known as a pathogen

The process of phagocytosis is shown in the above image.

associated molecular pattern (PAMP). PRRs on the surface of a phagocyte will bind to PAMPs on the surface of a pathogen, and the phagocyte will then ingest the pathogen to form a phagosome which is essentially a pocket in the cell that contains the pathogen. A lysosome is an organelle that contains digestive enzymes – more information on lysosomes can be found here. Lysosomes will fuse to the phagosome to form a phagolysosome, and the lysosome will release digestive enzymes into the phagolysosome which causes the pathogen to break down. The broken-down pathogen will then be discharged out of the cell as waste material.

Adaptive immunity provides a specific response and long-term immunity against a pathogen, and may require a few days before it kicks in. The adaptive response can be further separated into humoral immunity and cell-mediated immunity, which involve another type of WBC known as lymphocytes. B cells are components of humoral immunity, while T cells are components of cell-mediated immunity.

An antigen is a molecule that can be found on the surface of a pathogen or can be released by the pathogen itself. The B cell has two main functions – to produce antibodies against antigens or to present antigens to T cells. B cells can secrete antibody molecules that neutralise antigens and can also neutralise other toxins secreted by pathogens. To present antigens to T cells, the B cell receptor on the surface of a B cell recognises an antigen, which causes the B cell to phagocytose the pathogen. The B cell will then internalise and modify the pathogen and present the antigen on the B cell surface.

T cells can sometimes be referred to as T killer cells because, as the name suggests, they kill pathogens. B cells and other phagocytes can present an antigen on their surface to T cells which will activate the T cell and allow it to recognise the antigen as a target. When an activated T cell encounters a pathogen with the specific antigen on its surface, the T cell will create a pore in the pathogens surface. Through this pore, the T cell can release digesting enzymes directly into the pathogen, causing it to break down and be destroyed.

All in all, the immune system involves a very complex series of events and includes cellular and molecular components. In this post, I have only just scratched the surface on the roles of innate and adaptive immunity. There is much more to be written about, from the interaction between innate and adaptive immunity to immunotherapy. While the immune system is here to provide us with protection against infection, it does not always work, with some people being immunosuppressed or others suffering medical conditions as a result of autoimmunity.

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