Complement receptor 3 (CR3, CD11b/CD18) binds complement and is found on macrophages, neutrophils, and large granular lymphocytes. Complement receptor 4 (CR4, CD11c/CD18) also binds complement and is found on neutrophils, monocytes, and macrophages. Both CR3 and CR4 bind bacterial lipopolysaccharide and β-glucans and promote phagocytosis of unopsonized bacteria and yeast. Additionally, they are upregulated at sites of inflammation and enhance the binding of monocytes and neutrophils to endothelial cells, allowing diapedesis. Inherited deficiency of CR3 and CR4 occur in leukocyte adhesion deficiency type 1 (LAD-1).
LAD is a rare disease, affecting one in 1 million individuals. In this disorder, it is a failure to synthesize CD18, which then is associated with CD11b to form CR3 and with CD11c to form CR4. LFA-1 (CD11a/CD18), important in cellular adhesion and trafficking, is also deficient. These three receptor proteins are known as β-2 integrins, part of the Ig superfamily. LAD- 1 inheritance is autosomal recessive with some variability in the amounts of CD18 expressed, resulting in two phenotypes (severe deficiency and moderate deficiency). Omphalitis is a classic presenting infection with delayed separation of the umbilical cord. Infections from birth onward are usually seen with prominent bowel, perirectal, respiratory, and cutaneous infections. Recurrent and chronic bacterial infections are also seen. These are usually localized to the skin and mucosal surfaces and often involve Staphylococcus aureus and gram-negative bacilli. The characteristic pathologic finding is a lack of neutrophils at sites of infection and lymphoid tissues lacking lymphocytes. As expected, neutrophil counts in peripheral blood are usually elevated as a result of failure to recruit granulocytes to the sites of infection, a process critically dependent on functional CD18 as part of the β-2 integrins.
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