Immune Pathways Comparison

The alternative pathway of the complement system is bypassed by C1, C4, and C2 components.

Lectins initiate the classical pathway in the complement system.

C5a is involved in chemotaxis by activating and attracting neutrophils and macrophages.

The classical pathway belongs to innate immunity.

Mannose residues on the surface of glycoproteins initiate the classical pathway.

Anaphylatoxins like C3a, C4a, and C5a cause degranulation of mast cells.

The alternative pathway of complement activation is dependent on antibodies.

Complement proteins are designated with names like C1, C2, C3, etc.

The complement cascade involves the cleavage of molecules into fragments designated 'a' and 'b'.

Mannose-binding lectin (MBL) initiates the lectin pathway by binding to proteins on bacterial surfaces.

The activation of C1 results in the formation of C1s, which then activates C2.

The membrane attack complex (MAC) formation is bypassed in the classical pathway of complement activation.

C3 is directly activated through the properdin system in the absence of antibodies.

The letter 'a' in complement fragments designates the larger fragment.

Complement System Pathways

The classical pathway is initiated by antigen-antibody complexes and activates all components starting with C1, while the alternative pathway is initiated by microbial surface products and bypasses C1, C4, and C2, starting with C3.
The lectin pathway is initiated by mannose-binding lectin (MBL) and activates C2 and C4, then follows the classical pathway.

Biological Activities of Complement

Cell lysis occurs when the antigen is on the cell wall of bacteria or cells, and the membrane attack complex inserts into the cell membrane, disrupting it and causing lysis.
Opsonization occurs when cells and antigen-antibody complexes are coated with C3b, making them more susceptible to phagocytosis.
Inflammatory reactions are triggered by C5a, which attracts neutrophils and macrophages, and C3a, C4a, and C5a, which cause degranulation of mast cells and release of histamine and other inflammatory mediators.

Complement Activation

Complement is a complex enzymatic cascade involving over 20 proteins, which work in sequence to elicit a cascade of events.
The alternative pathway is the first to be produced and is independent of antibodies, attaching to the bacterial cell wall and cleaving C3.
Activation of the complement system involves the cleavage of complement molecules into fragments, with larger fragments designated by the letter "b" and smaller ones with the letter "a".

Compare and contrast the classical, alternative, and lectin pathways of the immune system, including their initiators, activation mechanisms, and specific characteristics.