Innate Immunity and the Complement System Overview

Innate Immunity and the Complement System

Epithelial Barriers

Epithelial barriers, such as skin and mucous membranes, serve as the primary line of defense against external threats like pathogens. They prevent entry into the body while allowing essential nutrients and oxygen to pass. Epithelial barriers secrete antimicrobial substances and maintain a balanced microbiota that competes with potential pathogens, thereby reducing infection risk.

Phagocytes

Phagocytes, such as macrophages and neutrophils, are white blood cells responsible for recognizing and engulfing invading pathogens. Once inside the phagocyte, they break down the pathogen using hydrolytic enzymes and present particles derived from the pathogen to helper T cells, initiating a secondary immune response.

Mast Cells

Mast cells reside near the endothelium and epithelium, particularly in areas susceptible to trauma and mucous membranes. When stimulated by allergens or pathogens, mast cells release histamine, heparin, serotonin, and other vasoactive amines, leading to vasodilation and increased vascular permeability, causing symptoms associated with inflammation and allergic reactions.

Basophils

Basophils are a type of granulocyte, similar in size and shape to mast cells. While less numerous than other granulocytes, basophils play a significant role in innate immune responses, particularly in counteracting pathogens and preventing severe allergic reactions. Upon encountering pathogens or allergens, basophils release histamine, leukotrienes, and prostaglandins, contributing to inflammation and eosinophil migration.

Eosinophils

Eosinophils are granulocytes with distinct granules staining orange with eosin. They are involved in the immune system's defense against parasitic infections and allergies. Eosinophils are attracted to sites of infection or inflammation by chemokines and secrete potent cytotoxic compounds, such as peroxidase enzymes, that kill the parasites.

Innate Lymphoid Cells

Innate lymphoid cells (ILCs) are a group of immune cells that do not require antigenic stimulation to become active. They are involved in the innate immune response and can produce cytokines, which are signaling proteins that help regulate immune responses. ILCs are classified into three main subtypes based on their expression of specific transcription factors: ILC1, ILC2, and ILC3.

ILC1

ILC1 cells are involved in the anti-viral immune response and are also involved in the destruction of cancer cells. They produce the cytokines interferon-gamma and tumor necrosis factor, which activate macrophages and cytotoxic T cells.

ILC2

ILC2 cells are involved in the immune response against helminths, such as intestinal parasites. They produce the cytokines interleukin-5, interleukin-13, and interleukin-22, which help to recruit and activate eosinophils, mast cells, and neutrophils.

ILC3

ILC3 cells are involved in the immune response against bacteria and to a lesser extent against viruses. They produce the cytokines interleukin-17 and interleukin-22, which are involved in the recruitment of neutrophils and the activation of mast cells.

In summary, the innate immune system, led by the complement system, employs various components, including epithelial barriers, phagocytes, mast cells, basophils, eosinophils, and innate lymphoid cells, to defend against a wide range of pathogens.