He was homozygous for a variant of the FCN3 gene, which encodes ficolin-3. A lack of ficolin-3 may cause immunodeficiency disease.
Ficolin (collagen/fibrinogen domain containing lectin) 2 (hucolin). Image source: Wikipedia, public domain.
Overview of the innate immune system (a mind map).
There are 3 pathways for activation of complement:
Mannose-binding lectin (MBL)
Mannose-binding lectin (MBL). Image source: Wikipedia.
The 3 pathways (CAM) converge at the point of cleavage of C3. Then C3b becomes an acceptor site for C5, and C5-9 form the membrane attacking complex (MAC). Thus, CAM leads to MAC:
CAM --> MAC
A complement protein attacking an invader. Image source: Wikipedia.
Classical and alternative complement pathways. Image source: Wikipedia.
Immunodeficiency Associated with FCN3 Mutation and Ficolin-3 Deficiency. NEJM, 2009.
Classical Pathway Complement System
Alternative Pathway Compliment System
The Complement System.
Figure 1 - The complement system as a bridge between innate and adaptive immunity. Nature.
The three major complement activation pathways
Overview of the Complement System