0.1 Module 2a & 2b: barrier defenses and the innate immune  (Page 3/23)

2) The granules of the NK cells release perforins. A perforin    is a protein that forms holes in the membranes of infected cells. This cause the cell to become leaky, losing important parts.

Both mechanisms are especially effective against virally infected cells. If apoptosis is induced before the virus has the ability to reprooduce, no additional infectious virus will be released from the cell, thus preventing further infection.

Recognition of pathogens

Cells of the innate immune response, the phagocytic cells, and the cytotoxic NK cells recognize patterns of specific pathogen molecules, such as bacterial cell wall components or bacterial proteins, using pattern recognition receptors. A pattern recognition receptor (PRR)    is a receptor that recognizes certain chemical features of a particular pathogen.

Should the cells of the innate immune system come into contact with a species of pathogen they recognize, the cell will bind to the pathogen and start phagocytosis or cellular apoptosis in an effort to destroy the offending microbe. Receptors vary somewhat according to cell type, but they usually include receptors for bacteria and for complement, discussed below.

Soluble mediators of the innate immune response

The previous discussions have alluded to chemical signals that can induce cells to change various physiological characteristics, such as the expression of a particular receptor. These soluble factors are secreted during innate or early induced responses, and later during adaptive immune responses.

Cytokines and chemokines

A cytokine    is signaling molecule that allows cells to communicate with each other over short distances. Cytokines are secreted into the intercellular (between the cells) space, and the action of the cytokine induces the receiving cell to change in order to fight the pathogen. A chemokine    is a chemical mediator similar to cytokines except that its function is to attract cells (chemotaxis) from longer distances.

Early induced proteins

Early induced proteins are those that are not always present in the body, but are made as they are needed early during the innate immune response. Interferons are an example of such proteins. Cells infected with viruses secrete interferons that travel to adjacent cells and induce them to make antiviral proteins. Thus, even though the initial cell is sacrificed, the surrounding cells are protected. Phagocytes such as macrophages have receptors for these proteins, and they are thus able to recognize them as they are bound to the bacteria. This brings the phagocyte and bacterium into close proximity and enhances the phagocytosis of the bacterium.

Complement system

The complement    system is a series of proteins always found in the blood plasma. As such, these proteins are not considered part of the early induced immune response    , Since they are always present. Made in the liver, they have a variety of functions in the innate immune response. Complement functions in the adaptive immune response in what is called the classical pathway. The complement system consists of several proteins that chemically alter and fragment later proteins in a series, which is why it is termed cascade. Once activated, the series of reactions is irreversible, and releases fragments that have the following actions: