**5. Description of the complement system:**

#### **5.1. Alternative pathway of the "complement system"**

We begin with the description of this pathway, which although referred to as "alternative" is phylogenetically older than the "classical pathway." It does not require the presence of antibodies (Abs) for activation, thus constituting an important defense in the early stages of infection, when there are no significant amounts of Ab synthesized. Continuously "at rest," it operates at a low level, and it is amplified in the presence of certain factors. So we can differentiate as follows:

#### **(a) Alternative pathway "resting," "idle," or "pacemaker"**

	- **a.** Factor H binds to C3b\*, attaching to the cytoplasmic membranes.
	- **b.** Factor I breaks the C3, displacing Factor H that returns intact to serum (would be ready to start its action again).
	- **c.** Factor I inactivates the free C3b bound to the cytoplasmic membrane itself (iC3b).
	- **d.** Factor I cleaves iC3b into C3c (small fragment in solution) and C3dg (inactive larger fragment bound to membrane).

#### **(b) "Amplification" of the alternative pathway ("positive feedback loop")**

preventing possible tissue damage [15, 16]. This set of molecules, those involved in the acti‐ vation and the regulators (distinguishing between "triggers"—those able to bypass con‐ trol systems—and "nontriggers"), is called the "complement system." The need for both "amplification" and "regulation" with strict control gives an idea of the complexity of the

We begin with the description of this pathway, which although referred to as "alternative" is phylogenetically older than the "classical pathway." It does not require the presence of antibodies (Abs) for activation, thus constituting an important defense in the early stages of infection, when there are no significant amounts of Ab synthesized. Continuously "at rest," it operates at a low level, and it is amplified in the presence of certain factors. So we can

(1) In normal plasma conditions (absence of infection), the internal thioester bond of the C3

as "C3iB." Factor B is equivalent to the C2 factor of the classical pathway detailed later.

(4) The C3iBb complex acts as a "C3 convertase" in fluid phase cleaving C3 into C3a and

(5) The "C3b\*" in fluid phase is hydrolyzed by water inactivating it. However, if by some chance the "C3b\*" bonds covalently to an external surface ("recognition of the strange"), the "amplification of the alternative pathway" would occur. It is said that "C3b \*"does not start this amplification within the body due to regulatory proteins that prevent it, such as

**b.** Factor I breaks the C3, displacing Factor H that returns intact to serum (would be ready

**d.** Factor I cleaves iC3b into C3c (small fragment in solution) and C3dg (inactive larger

**c.** Factor I inactivates the free C3b bound to the cytoplasmic membrane itself (iC3b).

O), also referred to as "C3i" ("tick‐over" or "idle" activation) **(Figure 3)**.

O)B complex, breaking fraction B and generating

O) forming

O)B complex, also referred to

fraction is spontaneously hydrolyzed in a low ratio with a water molecule (H<sup>2</sup>

O) or "C3i": It binds to factor B, forming the C3(H<sup>2</sup>

**a.** Factor H binds to C3b\*, attaching to the cytoplasmic membranes.

"Complement System."

156 A Comprehensive Review of Urticaria and Angioedema

differentiate as follows:

the complex C3(H<sup>2</sup>

(3) The D factor acting on the C3(H<sup>2</sup>

subproducts B1 and C3iBb.

to start its action again).

fragment bound to membrane).

(2) C3(H<sup>2</sup>

C3b\*.

the following:

**5. Description of the complement system:**

**5.1. Alternative pathway of the "complement system"**

**(a) Alternative pathway "resting," "idle," or "pacemaker"**


**Figure 3.** Alternative pathway activation of the complement system.

#### **5.2. Classical pathway of the "complement system"**

**1.** Activation of the complement system via the classical pathway requires the formation of the antigen‐antibody complex (Ag‐Ab), being the Ab of the subisotypes IgM, IgG1, IgG2, or IgG3. This interaction gives rise to conformational changes in the Fc fragment of immunoglobulin (Ig) generating an attachment site for the C1 fraction in the Cγ2 domain (constant part "2" of the IgG heavy chain) or the Cμ3 domain (constant part "3" of the IgM heavy chain).

	- **a.** It can bind to two or more IgG molecules through the Cγ2 domain when bound to the same Ag molecule (several IgG molecules are part of the same immunocomplex). IgG has only one binding site per molecule, so at least two IgG molecules are necessary to activate the complement system.
	- **b.** It can bind to two or more Cμ3 domains of different pentametric IgM subunits. The free pentametric IgM is "flat" but on binding to Ag, the Fab arms adopt angles with the Fc portions (in the "staple" configuration), and then C1q can bind to different monomers of the same pentametric IgM. The IgM exposes more adhesion sites when it is in "staple" configuration, explaining why the IgM is more likely to activate the complement system.
	- **a.** C1s bind to C4, producing two fragments: C4a (small fragment that diffuses into the plasma) and C4b (large fragment that binds to the membrane of the "target cell"). The C4a fraction is an "anaphylotoxin" that has importance later in this chain.
	- **b.** C1 finds a binding site on C4b, and like everything around C1s is cleaved into two fragments: C2a (large fragment attached to C4b) and C2b (small fragment that diffuses into the plasma).

**a.** The intact C3 fraction has a very stable internal thioester bond between a cysteine and a glutamine (product of posttranslational modification) whose half‐life is close to 600 h.

**2.** The C1 fraction of the complement system is composed of five subunits: a "C1q" subunit (stem with six helical arms, three copies of a fundamental unit in a "Y," which in turn consists of two groups of three chains each together form a triple helix), two C1r subunits (arranged resting on the two arms of C1q), and two C1s subunits (arranged resting on the two arms of C1q, whose catalytic domains are arranged toward the center), stabilized by

**3.** The C1q fraction is capable of binding to the Fc region of immunoglobulins provided they

**a.** It can bind to two or more IgG molecules through the Cγ2 domain when bound to the same Ag molecule (several IgG molecules are part of the same immunocomplex). IgG has only one binding site per molecule, so at least two IgG molecules are necessary to

**b.** It can bind to two or more Cμ3 domains of different pentametric IgM subunits. The free pentametric IgM is "flat" but on binding to Ag, the Fab arms adopt angles with the Fc portions (in the "staple" configuration), and then C1q can bind to different monomers of the same pentametric IgM. The IgM exposes more adhesion sites when it is in "staple" configuration, explaining why the IgM is more likely to activate the

**4.** Binding of multiple domains of the same C1 complex induces a conformational change that activates a "C1r" molecule by autocatalysis, which in turn activates the other "C1r" molecule. Once activated, the two "C1r" molecules exert hydrolysis of both C1s molecules

**5.** The binding of several globular domains of the same C1 complex appears to induce a con‐ formational change in this, which involves the activation of a C1r molecule by autocataly‐ sis; in turn, this activated C1r activates the other C1r molecule. The two active molecules exert C1r hydrolysis of the two C1s, whereby they are activated: the two active C1s possess

**6.** C1s has two substrates: C2 and C4. Note at this point the regulatory role of the C1 inhibi‐ tor (C1‐INH) molecule. A deficiency in this would result in uncontrolled activation of C1s acting on C2 and C4, with the consequent decrease in the levels of these two complement

**a.** C1s bind to C4, producing two fragments: C4a (small fragment that diffuses into the plasma) and C4b (large fragment that binds to the membrane of the "target cell"). The

**b.** C1 finds a binding site on C4b, and like everything around C1s is cleaved into two fragments: C2a (large fragment attached to C4b) and C2b (small fragment that diffuses

**7.** The C4bC2a complex (formed by the C2a and C4b bond) is called "C3 convertase" since it

C4a fraction is an "anaphylotoxin" that has importance later in this chain.

to be activated, which is when they possess serine esterase activity.

fractions that is apparent in patients with C1‐INH‐HAE:

activates C3 in fragments C3a and C3b (**Figure 5**):

the Ca++ cation.

form part of immunocomplexes, such that

158 A Comprehensive Review of Urticaria and Angioedema

activate the complement system.

complement system.

serine esterase activity (**Figure 4**).

into the plasma).


**Figure 4.** Classical pathway activation of the complement system, showing the binding of the C1q subunit to the Ig Fc, which is bound in turn to the cell membrane.

#### **5.3. Lectin pathway of the "complement system"**

The lectin pathway is a third way of complement system pathway activation different from the classical activation of C2 and C4 fractions (**Figure 6**). It starts with the action of the "man‐ nan‐binding protein" (MBP), which is structurally very similar to the C1q fraction (hexamers with 18 identical polypeptide chains coiled in groups of 3) and can bind two C1r subunits and two C1s subunits. However, it brings its own serine protease (called MASP) with 40% homology to C1r or C1s. MBP binds preferentially to the ends of mannose, fucose, and glu‐ cosamine of glycoproteins or polysaccharides present in the bacterial membrane. In a similar manner as described in the "classical pathway" with C1q2r2s complex, when MBP binds to carbohydrates it undergoes a conformational change, which in turn activates the serine pro‐ tease (MASP). Activated MASP acts sequentially on C2 and C4 fractions to produce the "C3 convertase of the classical pathway."

MASP‐1 has been recently shown to cleave bradykinin from HMWK [17] and its levels, together with the complex MASP1‐C1‐INH, have been related to disease severity in C1‐INH‐ HAE [18].

#### **5.4. Common final pathway of the "complement system"**

The three activation pathways of the complement system (the classical pathway, the alterna‐ tive pathway, and the lectin pathway) converge in a common final lytic pathway. The C5b, C6, C7, C8, and C9 fractions participate in the final lytic complement pathway and form a molecular structure known as "membrane attack complex" (MAC) (**Figure 7**).

**Figure 5.** Classical pathway for the activation of the complement system, where the formation of the C1qrs complex until the formation of the C3b molecule can be observed.

**Figure 6.** Lectin pathway for the activation of the complement system.

**5.3. Lectin pathway of the "complement system"**

160 A Comprehensive Review of Urticaria and Angioedema

convertase of the classical pathway."

the formation of the C3b molecule can be observed.

**5.4. Common final pathway of the "complement system"**

HAE [18].

The lectin pathway is a third way of complement system pathway activation different from the classical activation of C2 and C4 fractions (**Figure 6**). It starts with the action of the "man‐ nan‐binding protein" (MBP), which is structurally very similar to the C1q fraction (hexamers with 18 identical polypeptide chains coiled in groups of 3) and can bind two C1r subunits and two C1s subunits. However, it brings its own serine protease (called MASP) with 40% homology to C1r or C1s. MBP binds preferentially to the ends of mannose, fucose, and glu‐ cosamine of glycoproteins or polysaccharides present in the bacterial membrane. In a similar manner as described in the "classical pathway" with C1q2r2s complex, when MBP binds to carbohydrates it undergoes a conformational change, which in turn activates the serine pro‐ tease (MASP). Activated MASP acts sequentially on C2 and C4 fractions to produce the "C3

MASP‐1 has been recently shown to cleave bradykinin from HMWK [17] and its levels, together with the complex MASP1‐C1‐INH, have been related to disease severity in C1‐INH‐

The three activation pathways of the complement system (the classical pathway, the alterna‐ tive pathway, and the lectin pathway) converge in a common final lytic pathway. The C5b, C6, C7, C8, and C9 fractions participate in the final lytic complement pathway and form a

**Figure 5.** Classical pathway for the activation of the complement system, where the formation of the C1qrs complex until

molecular structure known as "membrane attack complex" (MAC) (**Figure 7**).

**Figure 7.** Major events in the lytic pathway cell membrane leading to the C9 polymerization and pore formation in the cell membrane.

MAC insertion into the cytoplasmic membrane causes an intercellular‐extracellular commu‐ nication pore with consequent ion exchange leading to cell death. The sequential steps are as follows:

	- **a.** The "classical pathway": the C4b2aC3b complex catalyzes the cleavage of C5 into C5a and C5b.
	- **b.** The "lectin pathway": the C4b2aC3b complex catalyzes the cleavage of C5 into C5a and C5b.
	- **c.** The "alternative pathway": a covalent attachment of a "new" C3b that forms part of the "C3 convertase," forming the C3bBb3b complex.
