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There are three pathways in complement system, alternative pathway, lectin pathway, and classical pathway.
Complement is a part of the innate immunity which serves as a first line defense against foreign body.

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- Macrophage has the function of phagocytosing pathogen. There are pattern recognition receptors (PRRs) to recognise  pathogen-associated molecular patterns (PAMPs).
- Macrophage also has the function of presenting antigen to T cells.

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B cells are present for the production of antibodies in humoral immunity. When it activated, it will form a plasma cells.

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CD 4 T cells can give rise to Th1 or Th2 cells depending on the cytokine environment.
CD 8 T cells are cytotoxic cell that kill infected cells by inducing apoptosis (involve perforin, granulysin, and granzymes).
Th1 cell is mainly associated with cell-mediated immunity whereas Th2 cell is mainly related to B cell proliferation and differentiation (humoral immunity).

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How is the complement pathway activated?
What is the mechanism of complement pathway?
What are the functions and outcome of complement pathway?

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What is the other function of macrophage?
Types of macrophage?
How macrophage kills pathogen?

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- Macrophages also function to produce cytokines that react differently on certain cells.
- There are 2 types of macrophage, M1 and M2. M1 leads to pro-inflammatory effects. For example, IL-1, IL-6 and TNF-alpha are the pro-inflammatory cytokines produced by M1 to induce fever for optimum immune response. M2 has an anti-inflammatory effect. For instance, IL-10, VEGF, and TGF-beta are functioning to repair damaged tissue.
- Macrophage kills pathogen by  produce inducible nitric oxide, reactive oxygen species.

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What is the function of the B cell?
How B cell become activated?

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How naive T cell is activated?
What is cross-priming?
Cytokine involved in differentiation of T cell?

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- The alternative pathway is activated by mainly bacterial endotoxin. Next, the lectin pathway is activated when there is the binding of the complex of mannose-binding lectin (MBL), ficolins, and MASP-1 and MASP-2 to PAMPs.  The classical pathway is activated by the detection of antibodies that are bound to the surface of microbes through a C1 protein.
- The complement pathway results in direct cytolysis via the formation of membrane attack complex, opsonisation where C3b binds and coat the pathogens for the macrophage to recognise and perform phagocytosis. Furthermore, the complement pathway also leads to chemotaxis to recruit neutrophils, eosinophils, monocytes and macrophages. Lastly, the result of inflammation.

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Function of B cell
- Opsonization
- Neutralization
- Complement
- Antibody production
- Antigen presenting cells
- Memory B cells

Activation of B cells
1. T-cell independent. B cell recognize the PAMPs on the antigen surface. Cross presentation will occur. B cell activated into plasma cells. The plasma cells will then produce antibody. This type of activation will not produce memory B cells.
2. T-cell dependent. It required T helper cells to activate into plasma cells. They will bind together via the MHC class II to MHC receptor and B cell will form a cytokine receptor. Cytokine will produced and B cell will proliferate into plasma cells. Plasma cell will produce antibodies and also memory B cells which can create protection for future antigen attack.

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- Naive T cell's activation begins when it binds to antigen presented in MHC on the antigen-presenting cell's surface. Then, there will be  CD28:B7 binding and  CD40: CD40L  co-stimulatory interactions.  CD40 signalling enhances the T cell activation; for instance,  upregulation of B7 expression on APCs and enhance IL-12 production. Lastly,  IL-2 binds to the IL-2 receptors on the T cells. Now, the T cell is activated.
- Cross-presentation is the ability of certain professional antigen-presenting cells o take up, process and present extracellular antigens via MHC class I molecules to CD8 T cells
- IL-12 and IFN-gamma are involved in Th1 cell differentiation whereas IL-4 is involved in Th2 cell differentiation.