Innate Immunity Study Notes

These notes cover the basics of the innate immune system, including its components, mechanisms, and differences from the adaptive immune system. The innate immune system acts as a fast-acting, non-specific defense against pathogens, utilizing cells like macrophages and neutrophils, and processes like phagocytosis and cytokine release.

The immune system can be viewed as a series of barriers to infection. The initial barriers are the skin and mucous membranes, which prevent many pathogens from entering the body. Once a pathogen breaches these barriers, it encounters the innate immune system, followed by the adaptive immune system if the infection persists.

The innate immune system is a fast-acting, non-specific defense mechanism. It includes cells such as macrophages and neutrophils, and proteins like complement. These components attack and attempt to kill pathogens immediately upon entry. However, the innate immune system may not be sufficient to control overwhelming infections or viral infections.

The adaptive immune system, composed of B cells and T cells, provides a highly specific response tailored to each invader. While slower to mount its response, the adaptive immune system is more powerful and possesses immunological memory, leading to a faster and stronger response upon subsequent encounters with the same pathogen.

A key difference between the innate and adaptive immune systems lies in antigen recognition. The innate system can be activated by free antigens, while the adaptive system, especially T cells, requires antigen presentation. This involves pathogens being engulfed by antigen-presenting cells (APCs), broken down, and presented to T cells for activation.

Cytokines are cell signaling proteins released by immune cells that stimulate an inflammatory response. Important subsets of cytokines include chemokines (attract immune cells), interleukins (mediate communication between leukocytes), tumor necrosis factors (TNF), transforming growth factors (TGF), and interferons (interfere with viral replication).

CD (Cluster of Differentiation) molecules are cellular surface markers used to identify cell types. For example, CD8 is found on a specific subtype of T cell. These molecules can also function as receptors and cell binding sites.

Phagocytes, including macrophages and neutrophils, are key players in the innate immune system. Macrophages are considered the hallmark cell of innate immunity, while neutrophils are recruited to assist macrophages during infection. Macrophages perform phagocytosis, cytokine production, and antigen presentation. Kupffer cells (liver), microglia (CNS), and osteoclasts (bone) are examples of tissue-resident macrophages.

Phagocytosis involves macrophages engulfing pathogens into phagosomes, which then merge with lysosomes containing deadly enzymes. These enzymes, along with reactive oxygen species and reactive nitrogen intermediates, kill the bacteria and viruses. Some pathogens, like Tuberculosis, block the fusion of the phagosome with the lysosome, allowing them to proliferate inside macrophages.

Neutrophils are derived from the bone marrow and circulate in the blood for approximately 5 days unless activated. They are drawn from the bloodstream to sites of inflammation through a process involving rolling (selectin ligand binding to endothelial selectins), crawling (integrin binding to ICAM), and transmigration (binding to PECAM-1). Chemokines like C5a and IL-8 guide neutrophils to the site of inflammation.

Pathogen-associated molecular patterns (PAMPs) are molecules present on many microbes but not usually found on human cells. The innate immune system recognizes PAMPs through pattern recognition receptors (PRRs) like Toll-like receptors (TLRs). Upon recognition, cells secrete cytokines to trigger an inflammatory response.

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